h264: improve checks before calling ff_h264_decode_extradata

  The ff_h264_decode_extradata routine now checks for the buffer size
and pointer internally. This makes it possible to remove the external
checks in ff_h264_decode_init.

  In decode_frame there was a size check missing because the buffer
gets tested prior to the invocation of ff_h264_decode_extradata().
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

libavcodec/h264.c

@@ -1083,8 +1083,7 @@ av_cold int ff_h264_decode_init(AVCodecContext *avctx){
         avctx->ticks_per_frame = 2;
     }
 
-    if(avctx->extradata_size > 0 && avctx->extradata &&
-        ff_h264_decode_extradata(h, avctx->extradata, avctx->extradata_size))
+    if(ff_h264_decode_extradata(h, avctx->extradata, avctx->extradata_size))
         return -1;
 
     if(h->sps.bitstream_restriction_flag && s->avctx->has_b_frames < h->sps.num_reorder_frames){
@@ -3918,7 +3917,7 @@ static int decode_frame(AVCodecContext *avctx,
 
         return 0;
     }
-    if(h->is_avc && AV_RB32(buf)==0x0164001F && buf[5] && buf[8]==0x67)
+    if(h->is_avc && buf_size >= 9 && AV_RB32(buf)==0x0164001F && buf[5] && buf[8]==0x67)
         return ff_h264_decode_extradata(h, buf, buf_size);
 
     buf_index=decode_nal_units(h, buf, buf_size);