avformat/utils: Improve ffio_limit logic

The earlier code would not complain if the remaining size was one byte
short of the desired size; and the way it performed the check could run
into signed integer overflow.

Fixes: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
Fixes: Timeout
Fixes: 26434/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-5752845451919360
Fixes: 26444/clusterfuzz-testcase-minimized-ffmpeg_dem_BINK_fuzzer-4697773380993024

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

libavformat/utils.c

@@ -253,9 +253,11 @@ int ffio_limit(AVIOContext *s, int size)
             remaining= FFMAX(remaining, 0);
         }
 
-        if (s->maxsize>= 0 && remaining+1 < size) {
-            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG, "Truncating packet of size %d to %"PRId64"\n", size, remaining+1);
-            size = remaining+1;
+        if (s->maxsize >= 0 && remaining < size && size > 1) {
+            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG,
+                   "Truncating packet of size %d to %"PRId64"\n",
+                   size, remaining + !remaining);
+            size = remaining + !remaining;
         }
     }
     return size;