The CONVERT_TZ function crashes the server when the
timezone argument is an empty SET field value.

1) The CONVERT_TZ may find a timezone string in the
   tz_names hash.
2) A string representation of the empty SET is a
   String of zero length with the NULL pointer.
3) If the key argument length is zero, hash functions
   do comparison using the length of the record being
   compared against.

I.e. a zero-length String buffer is an invalid
argument for hash search functions, and if String
points to NULL buffer, hashcmp() fails with SEGV
accessing that memory.

The my_tz_find function has been modified to
treat empty Strings as invalid timezone values
to skip unnecessary hash search.