avcodec/wbmpdec: use remaining size not whole size

Fixes: out of array access
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WBMP_fuzzer-6652634692190208
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WBMP_fuzzer-6653703453278208
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WBMP_fuzzer-6668020758216704
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WBMP_fuzzer-6684749875249152

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

libavcodec/wbmpdec.c

@@ -72,7 +72,7 @@ static int wbmp_decode_frame(AVCodecContext *avctx, AVFrame *p,
     if (p->linesize[0] == (width + 7) / 8)
         bytestream2_get_buffer(&gb, p->data[0], height * ((width + 7) / 8));
     else
-        readbits(p->data[0], width, height, p->linesize[0], gb.buffer, gb.buffer_end - gb.buffer_start);
+        readbits(p->data[0], width, height, p->linesize[0], gb.buffer, gb.buffer_end - gb.buffer);
 
     p->key_frame = 1;
     p->pict_type = AV_PICTURE_TYPE_I;