h264: ff_h264_decode_extradata: check buffer args

  The buffer size and pointer were not checked prior to testing the first
byte of the buffer. These were sometimes checked before calling, but it is
better to add it inside the function as it takes buf and size arguments.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

libavcodec/h264.c

@@ -995,6 +995,9 @@ int ff_h264_decode_extradata(H264Context *h, const uint8_t *buf, int size)
 {
     AVCodecContext *avctx = h->s.avctx;
 
+    if(!buf || size <= 0)
+        return -1;
+
     if(buf[0] == 1){
         int i, cnt, nalsize;
         const unsigned char *p = buf;