avcodec/4xm: Use av_unreachable() instead of av_assert0(0)

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

libavcodec/4xm.c

@@ -337,7 +337,8 @@ static inline void mcdc(uint16_t *dst, const uint16_t *src, int log2w,
         }
         break;
     default:
-        av_assert0(0);
+        av_unreachable("log2w starts at 3 and gets only decremented during "
+                       "recursive calls to decode_p_block");
     }
 }