avformat/ac4dec: Check remaining space in ac4_probe()

Fixes: CID1538298 Untrusted loop bound
Fixes: undefined behavior

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2f04cb673c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/ac4dec.c

@@ -42,6 +42,8 @@ static int ac4_probe(const AVProbeData *p)
             size += 4;
             if (buf[1] == 0x41)
                 size += 2;
+            if (left < size)
+                break;
             max_frames++;
             left -= size;
             buf += size;