avformat/iff: Saturate avio_tell() + 12

Fixes: signed integer overflow: 9223372036854775796 + 12 cannot be represented in type 'long long'
Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-4898373660704768

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

libavformat/iff.c

@@ -218,7 +218,7 @@ static int parse_dsd_diin(AVFormatContext *s, AVStream *st, uint64_t eof)
 {
     AVIOContext *pb = s->pb;
 
-    while (avio_tell(pb) + 12 <= eof && !avio_feof(pb)) {
+    while (av_sat_add64(avio_tell(pb), 12) <= eof && !avio_feof(pb)) {
         uint32_t tag      = avio_rl32(pb);
         uint64_t size     = avio_rb64(pb);
         uint64_t orig_pos = avio_tell(pb);
@@ -255,7 +255,7 @@ static int parse_dsd_prop(AVFormatContext *s, AVStream *st, uint64_t eof)
     int dsd_layout[6];
     ID3v2ExtraMeta *id3v2_extra_meta;
 
-    while (avio_tell(pb) + 12 <= eof && !avio_feof(pb)) {
+    while (av_sat_add64(avio_tell(pb), 12) <= eof && !avio_feof(pb)) {
         uint32_t tag      = avio_rl32(pb);
         uint64_t size     = avio_rb64(pb);
         uint64_t orig_pos = avio_tell(pb);