avformat/webmdashenc: Don't pass NULL to memcmp

Affects the FATE-tests webm-dash-manifest-unaligned-video-streams,
webm-dash-manifest and webm-dash-manifest-representations.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit a42c47b77f)

libavformat/webmdashenc.c

@@ -155,7 +155,8 @@ static int bitstream_switching(AVFormatContext *s, const AdaptationSet *as)
             !av_strstart(track_num->value, gold_track_num->value, NULL) ||
             gold_par->codec_id != par->codec_id ||
             gold_par->extradata_size != par->extradata_size ||
-            memcmp(gold_par->extradata, par->extradata, par->extradata_size)) {
+            (par->extradata_size > 0 &&
+             memcmp(gold_par->extradata, par->extradata, par->extradata_size))) {
             return 0;
         }
     }