avcodec/mjpegdec: Check for for the bitstream end in mjpeg_decode_scan_progressive_ac()

Fixes timeout
Fixes: 496/clusterfuzz-testcase-5805083497332736

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -1397,6 +1397,10 @@ static int mjpeg_decode_scan_progressive_ac(MJpegDecodeContext *s, int ss,
         int block_idx    = mb_y * s->block_stride[c];
         int16_t (*block)[64] = &s->blocks[c][block_idx];
         uint8_t *last_nnz    = &s->last_nnz[c][block_idx];
+        if (get_bits_left(&s->gb) <= 0) {
+            av_log(s->avctx, AV_LOG_ERROR, "bitstream truncated in mjpeg_decode_scan_progressive_ac\n");
+            return AVERROR_INVALIDDATA;
+        }
         for (mb_x = 0; mb_x < s->mb_width; mb_x++, block++, last_nnz++) {
                 int ret;
                 if (s->restart_interval && !s->restart_count)