mmcli: abort certain operations if modem is unlocked

Do not try to perform feature/capability specific actions if we're still
unlocked, as the modem didn't yet export the feature/capability specific
interface.

The only interfaces expected while modem is unlocked are the Modem and Simple
interfaces, as well as the SIM interface in the SIM object, of course.

cli/mmcli-modem-simple.c

@@ -125,11 +125,12 @@ context_free (Context *ctx)
 static void
 ensure_modem_simple (void)
 {
-    if (ctx->modem_simple)
-        return;
+    if (!ctx->modem_simple) {
+        g_printerr ("error: modem has no Simple capabilities\n");
+        exit (EXIT_FAILURE);
+    }
 
-    g_printerr ("error: modem has no Simple capabilities\n");
-    exit (EXIT_FAILURE);
+    /* Success */
 }
 
 void