UDF isn't supposed to use my_error(), it should return
the error message in the provided error message buffer

Fixes valgrind:

==93993== Conditional jump or move depends on uninitialised value(s)
==93993==    at 0x484ECCD: strnlen (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==93993==    by 0x1AD2B2C: process_str_arg (my_vsnprintf.c:259)
==93993==    by 0x1AD47E0: my_vsnprintf_ex (my_vsnprintf.c:696)
==93993==    by 0x1A3B91E: my_error (my_error.c:120)
==93993==    by 0xF87BE8: udf_handler::fix_fields(THD*, Item_func_or_sum*, unsigned int, Item**) (item_func.cc:3638)

followup for 267dd5a9