Bug 1402370 nullptr cannot be cast to bool, so return false instead of nullptr in GLBlitHelperD3D.cpp r=jrmuizel

MozReview-Commit-ID: EeboZBGD0Es

--HG--
extra : rebase_source : d339355bf5ffd6aa6d8e98f8a5c7ab4e3ec436cf

gfx/gl/GLBlitHelperD3D.cpp

@@ -261,7 +261,7 @@ GLBlitHelper::BlitDescriptor(const layers::SurfaceDescriptorD3D10& desc,
     if (format != gfx::SurfaceFormat::NV12) {
         gfxCriticalError() << "Non-NV12 format for SurfaceDescriptorD3D10: "
                            << uint32_t(format);
-        return nullptr;
+        return false;
     }
 
     const auto tex = OpenSharedTexture(d3d, handle);