bmv: remove unreachable default case

Fixes CID732191
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/bmv.c

@@ -187,8 +187,6 @@ static int decode_bmv_frame(const uint8_t *source, int src_len, uint8_t *frame,
                 memset(dst, val, len);
             }
             break;
-        default:
-            break;
         }
         if (dst == dst_end)
             return 0;