Ensuring that GstEnginePipeline decodebin is not dereferenced before its state is set to NULL.

Fixes #7115 which resulted from previous commit.

src/engines/gstelementdeleter.cpp

@@ -28,4 +28,5 @@ void GstElementDeleter::DeleteElementLater(GstElement* element) {
 
 void GstElementDeleter::DeleteElement(GstElement* element) {
   gst_element_set_state(element, GST_STATE_NULL);
+  gst_object_unref(element);
 }

src/engines/gstenginepipeline.cpp

@@ -1146,6 +1146,7 @@ void GstEnginePipeline::SourceSetupCallback(GstURIDecodeBin* bin,
 
 void GstEnginePipeline::TransitionToNext() {
   GstElement* old_decode_bin = uridecodebin_;
+  gst_object_ref(old_decode_bin);
 
   ignore_tags_ = true;