avformat/utils: Fix undefined NULL + 0

This is undefined behaviour in C, so use data = len ? data + len : data
instead of data += len. GCC optimizes the branch away in this case;
Clang unfortunately doesn't.

Fixes ticket #8592.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/utils.c

@@ -1426,7 +1426,7 @@ static int parse_packet(AVFormatContext *s, AVPacket *pkt,
         pkt->pts = pkt->dts = AV_NOPTS_VALUE;
         pkt->pos = -1;
         /* increment read pointer */
-        data += len;
+        data  = len ? data + len : data;
         size -= len;
 
         got_output = !!out_pkt.size;