[depth-limit] causes leaking of internal error when undefined fragment is referenced

Executing the following document:

```graphql
query {
...SomeNonExistingFragment
}
```

Results in the following error:

```
TypeError: Cannot read property 'kind' of undefined
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:60:16)
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:72:14)
at /app/node_modules/graphql-depth-limit/index.js:69:9
at Array.map ()
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:68:59)
at /app/node_modules/graphql-depth-limit/index.js:77:9
at Array.map ()
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:76:55)
at /app/node_modules/graphql-depth-limit/index.js:69:9
at Array.map ()
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:68:59)
at /app/node_modules/graphql-depth-limit/index.js:77:9
at Array.map ()
at determineDepth (/app/node_modules/graphql-depth-limit/index.js:76:55)
at /app/node_modules/graphql-depth-limit/index.js:22:27
at /app/node_modules/graphql/validation/validate.js:65:12
```

This is actually an issue with `graphql-depth-limit` - but given that it is not really maintained since 4 years we should maybe improve it and inline it?