avcodec/tiff: Enforce increasing offsets

This may break some valid tiff files, it appears the specification does not require
the offsets to be increasing. They increase in the 2 test files i have though except
the last offset which is 0 (an end marker) and for which a special case is added to
avoid asking for a sample for that end marker.

See: [FFmpeg-devel] [PATCH 2/2] avcodec/tiff: Detect infinite retry loop
for an alternative implementation

Fixes: Timeout (Infinite -> Finite)
Fixes: 15706/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TIFF_fuzzer-5114674904825856

This variant was requested by paul on IRC
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiff.c

@@ -1399,7 +1399,7 @@ static int decode_frame(AVCodecContext *avctx,
     TiffContext *const s = avctx->priv_data;
     AVFrame *const p = data;
     ThreadFrame frame = { .f = data };
-    unsigned off;
+    unsigned off, last_off;
     int le, ret, plane, planes;
     int i, j, entries, stride;
     unsigned soff, ssize;
@@ -1454,6 +1454,7 @@ again:
     /** whether we should process this multi-page IFD's next page */
     retry_for_page = s->get_page && s->cur_page + 1 < s->get_page;  // get_page is 1-indexed
 
+    last_off = off;
     if (retry_for_page) {
         // set offset to the next IFD
         off = ff_tget_long(&s->gb, le);
@@ -1463,6 +1464,14 @@ again:
     }
 
     if (retry_for_subifd || retry_for_page) {
+        if (!off) {
+            av_log(avctx, AV_LOG_ERROR, "Requested entry not found\n");
+            return AVERROR_INVALIDDATA;
+        }
+        if (off <= last_off) {
+            avpriv_request_sample(s->avctx, "non increasing IFD offset\n");
+            return AVERROR_INVALIDDATA;
+        }
         if (off >= UINT_MAX - 14 || avpkt->size < off + 14) {
             av_log(avctx, AV_LOG_ERROR, "IFD offset is greater than image size\n");
             return AVERROR_INVALIDDATA;