The reason of the failure was apparent flaw in that a pointer to an uninitialized buffer was
passed to DBUG_PRINT of Protocol_text::store().

Fixed with splitting the print-out into two branches: 
one with length zero of the problematic arg and the rest.


sql/protocol.cc:
  correcting DBUG_PRINT to print out `from' with a care because one may be uninitialized in the caller.