[BUG] Since openapi generator 5.1.0 cannot parse the 3GPP R16-July-2020 yamls for 5G (related to $ref)

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Since openapi generator 5.1.0, I cannot parse the 3GPP R16-July-2020 yamls for 5G.

To highlight what cannot be parsed, the file example_part1.yaml represents an extract of TS29504_Nudr_DR.yaml which contains only the api /subscription-data/{ueId}/{servingPlmnId}/provisioned-data/am-data and renammed /my-api.
The api /my-api in example_part1.yaml refers to the file example_part2.yaml in the same way as the api /subscription-data/{ueId}/{servingPlmnId}/provisioned-data/am-data in TS29504_Nudr_DR.yaml refers to TS29505_Subscription_Data.yaml and these sub dependencies.
example_part1.yaml and example_part2.yaml have be simplified to maximum possible to reproduce the problem.

With openapi generator cli 5.0.0, example_part1.yaml is well parsed (openapi_generator_cli_5.0.0 generate -g java -i example_part1.yaml -o tmp).
But this fails with the generator 5.1.0 (openapi_generator_cli_5.1.0 generate -g java -i example_part1.yaml -o tmp).
Also if I parse example_part2.yaml with the generator 5.1.0, it works.

Thanks for the help.

openapi-generator version

regression since openapi generator 5.1.0

OpenAPI declaration file content or url

example_part1.yaml

openapi: 3.0.0
info:
  version: '-'
  title: 'part1'
paths:
  /my-api:
    $ref: 'example_part2.yaml#/paths/~1my-api'

example_part2.yaml

openapi: 3.0.0
info:
  version: '-'
  title: 'part2'

paths:

  /my-api:
    get:
      responses:
        '200':
          description: Expected response to a valid request
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Schema1'
        default:
          description: Unexpected error
          content:
            application/problem+json:
              schema:
                type: string

components:
  schemas:
  
    Bytes:
      format: byte
      type: string

    RgWirelineCharacteristics:
      $ref: '#/components/schemas/Bytes'

    Schema2:
      type: object
      properties:
        rgWirelineCharacteristics:
          $ref: '#/components/schemas/RgWirelineCharacteristics'

    Schema1:
      $ref: '#/components/schemas/Schema2'

Generation Details

java -Dlog.level=debug -jar openapi-generator-cli-5.1.0.jar generate -g java -i example_part1.yaml -o tmp

[main] DEBUG o.o.codegen.config.GeneratorSettings - GeneratorSettings#build: %s
[main] DEBUG o.o.codegen.config.WorkflowSettings - WorkflowSettings#build: %s
[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.StringIndexOutOfBoundsException: begin 0, end -1, length 18
        at java.base/java.lang.String.checkBoundsBeginEnd(String.java:3319)
        at java.base/java.lang.String.substring(String.java:1874)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:113)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefSchema(ExternalRefProcessor.java:921)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processSchema(ExternalRefProcessor.java:200)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processProperties(ExternalRefProcessor.java:223)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processProperties(ExternalRefProcessor.java:230)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:157)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:117)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefSchemaObject(ExternalRefProcessor.java:869)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalPathItem(ExternalRefProcessor.java:280)
        at io.swagger.v3.parser.processors.PathsProcessor.processReferencePath(PathsProcessor.java:299)
        at io.swagger.v3.parser.processors.PathsProcessor.processPaths(PathsProcessor.java:63)
        at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:49)
        at io.swagger.v3.parser.OpenAPIV3Parser.resolve(OpenAPIV3Parser.java:175)
        at io.swagger.v3.parser.OpenAPIV3Parser.readContents(OpenAPIV3Parser.java:154)
        at io.swagger.v3.parser.OpenAPIV3Parser.readLocation(OpenAPIV3Parser.java:89)
        at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:523)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:432)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 3
Errors: 
        -begin 0, end -1, length 18
Warnings: 
        -begin 0, end -1, length 18

        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:546)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:432)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

Steps to reproduce

Related issues/PRs

Suggest a fix

Start by running the Java CLI command against example_part1.yaml and example_part2.yaml, comparing 5.0.0 and 5.1.0 behavior. Read the ExternalRefProcessor.processRefToExternalSchema stack-trace path and the surrounding OpenAPI parser calls. Done means the external path reference and its nested schema references generate successfully without the StringIndexOutOfBoundsException.