avcodec/mpegvideo_{dec,motion}: Mark unreachable code as unreachable

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpegvideo_dec.c

@@ -810,7 +810,7 @@ static inline void MPV_motion_lowres(MpegEncContext *s,
         }
         break;
     default:
-        av_assert2(0);
+        av_unreachable("No other mpegvideo MV types exist");
     }
 }
 

libavcodec/mpegvideo_motion.c

@@ -813,7 +813,8 @@ static av_always_inline void mpv_motion_internal(MpegEncContext *s,
             }
             break;
         }
-    default: av_assert2(0);
+    default:
+        av_unreachable("No other mpegvideo MV types exist");
     }
 }