backend/dummy: Warn about missing dummy mode specs

When malformed modes are provided and no valid mode spec is found, mutter
will eventually try to access the last element of an empty list. Warn about
this and exit properly.

https://gitlab.gnome.org/GNOME/mutter/-/issues/1481

src/backends/meta-monitor-manager-dummy.c

@@ -34,6 +34,7 @@
 #include "backends/meta-monitor.h"
 #include "backends/meta-monitor-config-manager.h"
 #include "backends/meta-output.h"
+#include "meta/main.h"
 #include "meta/util.h"
 
 #define MAX_MONITORS 5
@@ -190,6 +191,12 @@ append_monitor (MetaMonitorManager *manager,
         }
     }
 
+  if (!mode_specs)
+    {
+      g_warning ("Cannot create dummy output: No valid mode specs.");
+      meta_exit (META_EXIT_ERROR);
+    }
+
   for (l = mode_specs; l; l = l->next)
     {
       CrtcModeSpec *spec = l->data;