memcheck: Handle Err_ReallocSizeZero in MC_(eq_Error)

memcheck: Handle Err_ReallocSizeZero in MC_(eq_Error)

When an realloc size zero error is emitted MC_(eq_Error) is called to
see if the errors can be deduplicated. This crashed since
Err_ReallocSizeZero wasn't handled. Handle it like Err_Free.

Also add a testcase for this case and test with both
--realloc-zero-bytes-frees=yes and
--realloc-zero-bytes-frees=no.
Which will report a different number of errors.

https://bugs.kde.org/show_bug.cgi?id=470520