Bug #52304 valgrind does not like to print un-inited string in Protocol_text::store()

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

@@ -850,7 +850,7 @@ bool Protocol_text::store(const char *from, size_t length,
   CHARSET_INFO *tocs= this->thd->variables.character_set_results;
 #ifndef DBUG_OFF
   DBUG_PRINT("info", ("Protocol_text::store field %u (%u): %s", field_pos,
-                      field_count, from));
+                      field_count, (length == 0? "" : from)));
   DBUG_ASSERT(field_pos < field_count);
   DBUG_ASSERT(field_types == 0 ||
 	      field_types[field_pos] == MYSQL_TYPE_DECIMAL ||