huffyuv: do not abort on unknown pix_fmt; instead, return an error.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/huffyuv.c

@@ -514,7 +514,7 @@ s->bgr32=1;
         }
         break;
     default:
-        assert(0);
+        return AVERROR_INVALIDDATA;
     }
 
     alloc_temp(s);