[C++] Undefined behaviour in exception handling

There is a potential null pointer dereference here: https://github.com/antlr/antlr3/blob/master/runtime/Cpp/include/antlr3exception.inl#L315

m_expectingSet is very often null. It should be checked before doing anything with this pointer.

Currently it leads to undefined behaviour and sometimes crashes of the whole program.

Open runtime/Cpp/include/antlr3exception.inl at line 315 and inspect how m_expectingSet is used during exception handling. Confirm when the pointer is null and verify that the handling path no longer dereferences it or produces undefined behavior; the issue provides no test file, so identify an appropriate runtime regression test if the repository has one.

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