libs: decoder: Don't unref null object
** (gst-launch-1.0:9789): CRITICAL **: 15:29:09.330: gst_vaapi_context_unref: assertion 'context != NULL' failed
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** (gst-launch-1.0:9789): CRITICAL **: 15:29:09.330: gst_vaapi_context_unref: assertion 'context != NULL' failed