avformat/dfa: check first frame offset in probe

Fixes probetest failure
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/dfa.c

@@ -28,6 +28,9 @@ static int dfa_probe(AVProbeData *p)
     if (p->buf_size < 4 || AV_RL32(p->buf) != MKTAG('D', 'F', 'I', 'A'))
         return 0;
 
+    if (AV_RL32(p->buf + 16) != 0x80)
+        return AVPROBE_SCORE_MAX / 4;
+
     return AVPROBE_SCORE_MAX;
 }