ozone: Ensure we don't return invalid GbmBuffer objects

In this particular case, ensure we don't return a non-null GbmBuffer
backed by a null gbm_bo.

Bug: 882429
Change-Id: I4f54102f18d6c177984316b07f30a1f1e4da92ff
Reviewed-on: https://chromium-review.googlesource.com/1219087
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593764}

ui/ozone/common/linux/gbm_wrapper.cc

@@ -218,7 +218,11 @@ class Device final : public ui::GbmDevice {
       gbm_flags &= ~GBM_BO_USE_SCANOUT;
 
     struct gbm_bo* bo = nullptr;
-    if (gbm_device_is_format_supported(device_, format, gbm_flags)) {
+    if (!gbm_device_is_format_supported(device_, format, gbm_flags)) {
+      LOG(ERROR) << "gbm format not supported: " << format;
+      return nullptr;
+    }
+
     struct gbm_import_fd_planar_data fd_data;
     fd_data.width = size.width();
     fd_data.height = size.height();
@@ -239,7 +243,6 @@ class Device final : public ui::GbmDevice {
       LOG(ERROR) << "nullptr returned from gbm_bo_import";
       return nullptr;
     }
-    }
 
     return std::make_unique<Buffer>(bo, format, gbm_flags, planes[0].modifier,
                                     std::move(fds), size, std::move(planes));