Fixed access to uninitalized memory found by valgrind

sql/sql_repl.cc

@@ -1958,7 +1958,7 @@ send_event_to_slave(binlog_send_info *info, Log_event_type event_type,
 
   pos= my_b_tell(log);
   if (repl_semisync_master.update_sync_header(info->thd,
-                                              (uchar*) packet->c_ptr(),
+                                              (uchar*) packet->c_ptr_safe(),
                                               info->log_file_name + info->dirlen,
                                               pos, &need_sync))
   {
@@ -1982,7 +1982,8 @@ send_event_to_slave(binlog_send_info *info, Log_event_type event_type,
     }
   }
 
-  if (need_sync && repl_semisync_master.flush_net(info->thd, packet->c_ptr()))
+  if (need_sync && repl_semisync_master.flush_net(info->thd,
+                                                  packet->c_ptr_safe()))
   {
     info->error= ER_UNKNOWN_ERROR;
     return "Failed to run hook 'after_send_event'";