v4l2videodec: Clear the input state pointer after unref

If caps are set again, we have a risk od returning from set_format with a
input_state pointing to dead memory. Clearing the pointer after unref fix
this issue.

sys/v4l2/gstv4l2videodec.c

@@ -251,6 +251,7 @@ gst_v4l2_video_dec_set_format (GstVideoDecoder * decoder,
       goto done;
     }
     gst_video_codec_state_unref (self->input_state);
+    self->input_state = NULL;
 
     /* FIXME we probably need to do more work if pools are active */
   }