avcodec/h264dec: check number of SPS in is_extra

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

libavcodec/h264dec.c

@@ -855,6 +855,8 @@ static int is_extra(const uint8_t *buf, int buf_size)
 {
     int cnt= buf[5]&0x1f;
     const uint8_t *p= buf+6;
+    if (!cnt)
+        return 0;
     while(cnt--){
         int nalsize= AV_RB16(p) + 2;
         if(nalsize > buf_size - (p-buf) || (p[2] & 0x9F) != 7)