avformat/imx: Check palette chunk size

Fixes: out of array write
Fixes: 32116/clusterfuzz-testcase-minimized-ffmpeg_dem_SIMBIOSIS_IMX_fuzzer-6702533894602752

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 f7a5150447)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/imx.c

@@ -113,6 +113,8 @@ retry:
             imx->first_video_packet_pos = pos;
         break;
     case 0xAA98:
+        if (chunk_size > 256 * 3)
+            return AVERROR_INVALIDDATA;
         for (int i = 0; i < chunk_size / 3; i++) {
             unsigned r = avio_r8(pb) << 18;
             unsigned g = avio_r8(pb) << 10;