Exception from Tool while parsing an ANTLR grammar

this particular case causes antlr4 to pop out "can't find rule grammarSpec or tree structure error" without the mutual recursion error that identifies the rules in violation. Commenting out the recursion in rule2 allows for the mutual recursion error to surface.

```
grammar T;

rule1:
rule2
| rule3
;

rule2:
rule1
| rule2
;

rule3:
rule1
| rule3
;
```

gets exception

```
can't find rule grammarSpec or tree structure error: (COMBINED_GRAMMAR T (RULES (RULE rule1 (BLOCK (ALT rule2) (ALT rule3))) (RULE rule2 (BLOCK (ALT rule1) (ALT rule2))) (RULE rule3 (BLOCK (ALT rule1) (ALT rule3)))))
...
Caused by: java.lang.ArrayIndexOutOfBoundsException: 2
at org.antlr.v4.semantics.SymbolCollector.discoverOuterAlt(SymbolCollector.java:111)
at org.antlr.v4.parse.GrammarTreeVisitor.outerAlternative(GrammarTreeVisitor.java:2285)
at org.antlr.v4.parse.GrammarTreeVisitor.ruleBlock(GrammarTreeVisitor.java:2198)
at org.antlr.v4.parse.GrammarTreeVisitor.rule(GrammarTreeVisitor.java:1664)
at org.antlr.v4.parse.GrammarTreeVisitor.rules(GrammarTreeVisitor.java:1242)
at org.antlr.v4.parse.GrammarTreeVisitor.grammarSpec(GrammarTreeVisitor.java:481)
... 12 more
Exception in thread "main" java.lang.NullPointerException
at org.antlr.v4.tool.Rule.getAltLabels(Rule.java:217)
at org.antlr.v4.tool.LeftRecursiveRule.getAltLabels(LeftRecursiveRule.java:90)
at org.antlr.v4.tool.Rule.hasAltSpecificContexts(Rule.java:200)
at org.antlr.v4.tool.LeftRecursiveRule.hasAltSpecificContexts(LeftRecursiveRule.java:61)
at org.antlr.v4.semantics.SymbolChecks.checkForLabelConflicts(SymbolChecks.java:157)
at org.antlr.v4.semantics.SymbolChecks.process(SymbolChecks.java:105)
at org.antlr.v4.semantics.SemanticPipeline.process(SemanticPipeline.java:107)
at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:397)
at org.antlr.v4.Tool.process(Tool.java:384)
at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:343)
at org.antlr.v4.Tool.main(Tool.java:190)
```

Reproduce the failure with the supplied grammar, then inspect org.antlr.v4.semantics.SymbolCollector.discoverOuterAlt and the GrammarTreeVisitor rule-processing paths named in the stack trace. Confirm that mutual recursion is reported instead of the ArrayIndexOutOfBoundsException, and that the later NullPointerException does not occur.

Written by the indexing model from the issue text.