avcodec/jpeg2000_parser: Check state!=0

Fixes: out of array read
Fixes: 37664/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5893420460146688

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

libavcodec/jpeg2000_parser.c

@@ -142,7 +142,7 @@ static int find_frame_end(JPEG2000ParserContext *m, const uint8_t *buf, int buf_
             m->in_codestream = 0;
         } else if (m->in_codestream && (state & 0xFFFF) == 0xFF90) { // Are we in tile part header?
             m->read_tp = 8;
-        } else if (pc->frame_start_found && info_marker((state & 0xFFFF0000)>>16) && m->in_codestream) {
+        } else if (pc->frame_start_found && info_marker((state & 0xFFFF0000)>>16) && m->in_codestream && (state & 0xFFFF)) {
             // Calculate number of bytes to skip to get to end of the next marker.
             m->skip_bytes = (state & 0xFFFF)-1;