nmcli: be less insistant on exiting when readline() gets no input

When the input ends, we indeed eventually want to shut down.

Nevertheless, it might be that we terminated the input *because* we're
already shutting down and want do do our cleanup. Let's not take the
shortcut to nmc_exit() in case the main loop is no longer running.

This doesn't affect existing uses of nmc_readline(), but will be useful
in a future patch.

src/nmcli/common.c

@@ -929,7 +929,8 @@ read_again:
         }
     } else if (!rl_string) {
         /* Ctrl-D, exit */
-        nmc_exit();
+        if (g_main_loop_is_running(loop))
+            nmc_exit();
     }
 
     /* Return NULL, not empty string */