[mutter] renderer-native: Don't crash if the FB surface can't be locked

[Marco Trevisan <marcotrevi src gnome org>]

commit 0332b7394e56f71c15692663ab05f8c84a4b9eff
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Tue May 8 22:36:33 2018 +0100

    renderer-native: Don't crash if the FB surface can't be locked

 src/backends/native/meta-renderer-native.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/backends/native/meta-renderer-native.c b/src/backends/native/meta-renderer-native.c
index 2571cac25..cb8e03abe 100644
--- a/src/backends/native/meta-renderer-native.c
+++ b/src/backends/native/meta-renderer-native.c
@@ -1601,6 +1601,12 @@ gbm_get_next_fb_id (MetaGpuKms         *gpu_kms,
   /* Now we need to set the CRTC to whatever is the front buffer */
   next_bo = gbm_surface_lock_front_buffer (gbm_surface);
 
+  if (!next_bo)
+    {
+      g_error ("Impossible to lock surface front buffer: %m");
+      return FALSE;
+    }
+
   for (i = 0; i < gbm_bo_get_plane_count (next_bo); i++)
     {
       strides[i] = gbm_bo_get_stride_for_plane (next_bo, i);