vrend: Don't try to deref gbm if it was not initialized

minigbm allocations being enabled does not imply that GBM is also
initialized, e.g. if the renderer uses a GLX context, so don't try
to dereference gbm.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: John Bates <jbates@chromium.org>

src/vrend_renderer.c

@@ -10949,7 +10949,7 @@ static void vrend_renderer_fill_caps_v2(int gl_ver, int gles_ver,  union virgl_c
    }
 
 #ifdef ENABLE_MINIGBM_ALLOCATION
-   if (has_feature(feat_memory_object) && has_feature(feat_memory_object_fd)) {
+   if (gbm && has_feature(feat_memory_object) && has_feature(feat_memory_object_fd)) {
          if (!strcmp(gbm_device_get_backend_name(gbm->device), "i915") &&
              !vrend_winsys_different_gpu())
             caps->v2.capability_bits |= VIRGL_CAP_ARB_BUFFER_STORAGE;