avcodec/cbs_jpeg: Check length for SOS

Fixes: out of array access
Fixes: 19734/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5673507031875584
Fixes: 19353/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5703944462663680

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1812352d76)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_jpeg.c

@@ -197,6 +197,9 @@ static int cbs_jpeg_split_fragment(CodedBitstreamContext *ctx,
         if (marker == JPEG_MARKER_SOS) {
             length = AV_RB16(frag->data + start);
 
+            if (length > end - start)
+                return AVERROR_INVALIDDATA;
+
             data_ref = NULL;
             data     = av_malloc(end - start +
                                  AV_INPUT_BUFFER_PADDING_SIZE);