avcodec/cbs_jpeg: Fix uninitialized end index in cbs_jpeg_split_fragment()

Fixes: Out of array read
Fixes: 24043/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5084566275751936.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4a10bc8f6f5d600c44ecb9b43cd9abf13bf3bfae)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_jpeg.c

@@ -149,6 +149,7 @@ static int cbs_jpeg_split_fragment(CodedBitstreamContext *ctx,
             break;
         } else if (marker == JPEG_MARKER_SOS) {
             next_marker = -1;
+            end = start;
             for (i = start; i + 1 < frag->data_size; i++) {
                 if (frag->data[i] != 0xff)
                     continue;