gluploadelement: Avoid race condition of base class' context.

The base class' context may change, we should use the common API
with lock to access it, rather than the just directly access the
struct field.

Part-of: <gstreamer/gst-plugins-base!916>

ext/gl/gstgluploadelement.c

@@ -167,7 +167,7 @@ _gst_gl_upload_element_transform_caps (GstBaseTransform * bt,
   if (base_filter->display && !gst_gl_base_filter_find_gl_context (base_filter))
     return NULL;
 
-  context = GST_GL_BASE_FILTER (bt)->context;
+  context = gst_gl_base_filter_get_gl_context (base_filter);
 
   GST_OBJECT_LOCK (upload);
   if (upload->upload == NULL) {
@@ -191,7 +191,10 @@ _gst_gl_upload_element_transform_caps (GstBaseTransform * bt,
 
   ret_caps =
       gst_gl_upload_transform_caps (ul, context, direction, caps, filter);
+
   gst_object_unref (ul);
+  if (context)
+    gst_object_unref (context);
 
   return ret_caps;
 }