avformat/icodec: Fix crash probing fuzzed file

Avoid invalid memory read/crash when frame offset >= 0xfffffff8.
Base64-encoded example: AAABADAwMDAwMAAAMAAwMDAw/P///w==
(The previous commit verifies that p->buf_size >= 22.)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/icodec.c

@@ -63,7 +63,7 @@ static int probe(AVProbeData *p)
         offset = AV_RL32(p->buf + 18 + i * 16);
         if (offset < 22)
             return FFMIN(i, AVPROBE_SCORE_MAX / 4);
-        if (offset + 8 > p->buf_size)
+        if (offset > p->buf_size - 8)
             continue;
         if (p->buf[offset] != 40 && AV_RB64(p->buf + offset) != PNGSIG)
             return FFMIN(i, AVPROBE_SCORE_MAX / 4);