avformat/3dostr: Check remaining buffer in probe before 8 byte step

Fixes: segfault
Fixes: signal_sigabrt_7ffff6ae7cc9_7213_0d6457b9d6897fa7c78507fa5de53510.ts
Regression since: 3ac45bf665

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/3dostr.c

@@ -29,10 +29,9 @@ static int threedostr_probe(const AVProbeData *p)
         unsigned chunk = AV_RL32(p->buf + i);
         unsigned size  = AV_RB32(p->buf + i + 4);
 
-        i += 8;
         if (size < 8 || p->buf_size - i < size)
             return 0;
-
+        i += 8;
         size -= 8;
         switch (chunk) {
         case MKTAG('C','T','R','L'):