avformat/mvdec: Allocate extradata only once

Fixes: memleak
Fixes: 28686/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5822961932173312

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mvdec.c

@@ -213,10 +213,12 @@ static int parse_video_var(AVFormatContext *avctx, AVStream *st,
         st->codecpar->width = var_read_int(pb, size);
     } else if (!strcmp(name, "ORIENTATION")) {
         if (var_read_int(pb, size) == 1101) {
-            st->codecpar->extradata      = av_strdup("BottomUp");
-            if (!st->codecpar->extradata)
-                return AVERROR(ENOMEM);
-            st->codecpar->extradata_size = 9;
+            if (!st->codecpar->extradata) {
+                st->codecpar->extradata = av_strdup("BottomUp");
+                if (!st->codecpar->extradata)
+                    return AVERROR(ENOMEM);
+                st->codecpar->extradata_size = 9;
+            }
         }
     } else if (!strcmp(name, "Q_SPATIAL") || !strcmp(name, "Q_TEMPORAL")) {
         var_read_metadata(avctx, name, size);