ATN-failover with erroneous input can corrupt cached DFA

[sharwell]

When execDFA fails to find a cached edge for an input, it falls back to execATN. The fallback is enclosed in a try/catch block, and if execATN throws a RecognitionException an ERROR edge is added to the cached DFA. When the erroneous symbol that resulted in execATN throwing an exception is not the first symbol checked by execATN, the update to the cached DFA blocks the entire input sequence leading to the erroneous symbol starting with the first uncached (yet possibly valid) symbol. This is fixed in sharwell/antlr4@cd3adb1, and included in pull request parrt#15.

[sharwell]

I don't have a unit test for this issue currently. It arose periodically when I was doing permutation testing on the input by injecting thousands of different errors at different locations.