causes out of memory errors

The code in mysql_create_function() and mysql_drop_function() assumed
that the only reason for UDFs being uninitialized at that point is an
out-of-memory error during initialization. However, another possible 
reason for that is the --skip-grant-tables option in which case UDF 
initialization is skipped and UDFs are unavailable.

The solution is to check whether mysqld is running with
--skip-grant-tables and issue a proper error in such a case.