In function log_event.cc:Query_log_event::write, there was a cast that
was triggering undefined behavior. The offending cast is the
following:

  write_str_with_code_and_len((char **)(&start),
                              catalog, catalog_len, Q_CATALOG_NZ_CODE);

This results in calling write_str_with_code_and_len with first
argument pointing to a (char **) while "start" is itself a pointer to
uchar (uchar *). Inside write_str_with_..., the content of start is
then be updated:

  (*dst)+= len;

The instruction above would cause the (*dst) pointer (ie, the "start"
argument, from the caller point of view, and which actually points to
uchar instead of pointing to char) to be updated so that it would
increment catalog_len. However, this seems to break strict-aliasing
rules ultimately causing the increment and assignment to behave
unexpectedly.

We fix this by removing the cast and by making the types match.