h264: skip start code search if the size of the nal unit is known

Start code emulation prevention is only required in Annex B bytestream
packed NAL units. For other coding formats the size is already known.
Looking for a start code prefix can result in false positives like in
http://streams.videolan.org/streams/mp4/Mr_MrsSmith-h264_aac.mp4
which has a false positive in the SPS.

libavcodec/h264_parser.c

@@ -144,7 +144,7 @@ static inline int parse_nal_units(AVCodecParserContext *s,
                 src_length = 20;
             break;
         }
-        ptr= ff_h264_decode_nal(h, buf, &dst_length, &consumed, src_length);
+        ptr= ff_h264_decode_nal(h, buf, &dst_length, &consumed, src_length, 0);
         if (ptr==NULL || dst_length < 0)
             break;