Disable new connections in case of fatal signal

A user reported that MariaDB server got a signal 6 but still accepted new
connections and did not crash.  I have not been able to find a way to
repeat this or find the cause of issue. However to make it easier to
notice that the server is unstable, I added code to disable new
connections when the handle_fatal_signal() handler has been called.

sql/signal_handler.cc

@@ -149,6 +149,7 @@ extern "C" sig_handler handle_fatal_signal(int sig)
     goto end;
   }
   segfaulted = 1;
+  abort_loop= 1;                                // Disable now connections
   DBUG_PRINT("error", ("handling fatal signal"));
 
   curr_time= my_time(0);