avcodec/bethsoftvideo: Check block_type

Fixes: Timeout (17 seconds -> 1 second)
Fixes: 13184/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BETHSOFTVID_fuzzer-5711446296494080

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bethsoftvideo.c

@@ -109,6 +109,11 @@ static int bethsoftvid_decode_frame(AVCodecContext *avctx,
             if(yoffset >= avctx->height)
                 return AVERROR_INVALIDDATA;
             dst += vid->frame->linesize[0] * yoffset;
+        case VIDEO_P_FRAME:
+        case VIDEO_I_FRAME:
+            break;
+        default:
+            return AVERROR_INVALIDDATA;
     }
 
     // main code