gpu-kms: Protect against NULL crtc->current_mode

When failing to apply the CRTC mode, Mutter throws a warning
message. This warning message would try to access the name of
the CRTC's current mode.

There are ocasions, however, where the current mode is NULL.
On these cases, Mutter would crash.

Fix that by protecting against NULL CRTC modes.

src/backends/native/meta-gpu-kms.c

@@ -148,7 +148,10 @@ meta_gpu_kms_apply_crtc_mode (MetaGpuKms *gpu_kms,
                       connectors, n_connectors,
                       mode) != 0)
     {
-      g_warning ("Failed to set CRTC mode %s: %m", crtc->current_mode->name);
+      if (crtc->current_mode)
+        g_warning ("Failed to set CRTC mode %s: %m", crtc->current_mode->name);
+      else
+        g_warning ("Failed to set CRTC mode: %m");
       g_free (connectors);
       return FALSE;
     }