avcodec/magicyuv: Check bits left in flags&1 branch

Fixes: Timeout
Fixes: 8690/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MAGICYUV_fuzzer-6542020913922048

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

libavcodec/magicyuv.c

@@ -240,6 +240,8 @@ static int magy_decode_slice10(AVCodecContext *avctx, void *tdata,
 
         dst = (uint16_t *)p->data[i] + j * sheight * stride;
         if (flags & 1) {
+            if (get_bits_left(&gb) < bps * width * height)
+                return AVERROR_INVALIDDATA;
             for (k = 0; k < height; k++) {
                 for (x = 0; x < width; x++)
                     dst[x] = get_bits(&gb, bps);
@@ -368,6 +370,8 @@ static int magy_decode_slice(AVCodecContext *avctx, void *tdata,
 
         dst = p->data[i] + j * sheight * stride;
         if (flags & 1) {
+            if (get_bits_left(&gb) < 8* width * height)
+                return AVERROR_INVALIDDATA;
             for (k = 0; k < height; k++) {
                 for (x = 0; x < width; x++)
                     dst[x] = get_bits(&gb, 8);