Mfxdecoder: gst_mfx_decoder_flush: report null GstMfxDecoder instead of crashing.
Mfxdecoder: gst_mfx_decoder_flush: report null GstMfxDecoder instead of crashing.

Fixes #10

Signed-off-by: Xavier Hallade <93834dd5d35ebffc5a3e7d620f081db524e72207@intel.com>
Signed-off-by: Lim Siew Hoon <e412356d7e67dbdde94e841e5d46937fb9f4e623@intel.com>

rislim/gstreamer-media-SDK,ph0b/gstreamer-media-SDK,ishmael1985/gstreamer-media-SDK,rislim/gstreamer-media-SDK