The DBUG code emits the current value of the proc_info member of THD,
which may be set to NULL.  It was wrong to dereference that value
with the format string %s without verifying that it was valid.

Now, insert an inline test that substitutes the string "(null)" for
NULL pointers.


sql/sql_class.cc:
  Dereferencing a NULL is illegal (though not fatal for %s on some 
  platforms), and we have no assurance that the caller didn't call us 
  with a valid string.