fix invalid free of internal vector storage in replaceChildren

[zayeem06]

replaceChildren builds a temporary vector with antlr3VectorNew(1) when the replacement tree is not a nil node, and for any size hint at or below ANTLR3_VECTOR_INTERNAL_SIZE antlr3SetVectorApi points elements at the vector's own inline internal[] array instead of a separate heap block, so the cleanup path hands free() an interior pointer into the vector struct. Any tree rewrite that replaces children with a non-nil node hits it; under ASAN it aborts with "attempting free on address which was not malloc()-ed ... 16 bytes inside of 352-byte region" at antlr3basetree.c:454. The same block also assigned 0 to newChildren->size, which is the vector's size() function pointer rather than a count field. Calling the vector's own free() instead is the fix, since it already guards the elements release with elementsSize > ANTLR3_VECTOR_INTERNAL_SIZE; worth verifying that the replacement nodes still survive, which they do because they were added with a NULL freeptr, so the original "will not free the nodes" behaviour is unchanged.