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The parser crashes because it invokes GetBuffer inside
Parser, when Python is providing the next chunk of input,
which reallocs the buffer to a different location. However,
eventPtr is not updated inside GetBuffer (and neither is
positionPtr). As a result, the next access to eventPtr (in
XML_GetCurrentLineNumber, invoked from set_error), will
cause a segfault.

It is not clear to me why these pointers are not adjusted
when the buffer is reallocated. However, a consistent fix
appears to be to update the eventPtr close to the place
where positionPtr is initialized, which is done in the
attached patch exp.diff, which fixes this test case.

Fred, can you please review this patch?