avformat/imfdec: Simplify get_next_track_with_minimum_timestamp()

This also makes the code more robust

Fixes: CID1512414 Uninitialized pointer read

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Pierre-Anthony Lemieux <pal@sandflow.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f10493f6fc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/imfdec.c

@@ -700,12 +700,9 @@ static int imf_read_header(AVFormatContext *s)
 static IMFVirtualTrackPlaybackCtx *get_next_track_with_minimum_timestamp(AVFormatContext *s)
 {
     IMFContext *c = s->priv_data;
-    IMFVirtualTrackPlaybackCtx *track;
+    IMFVirtualTrackPlaybackCtx *track = NULL;
     AVRational minimum_timestamp = av_make_q(INT32_MAX, 1);
 
-    if (!c->track_count)
-        return NULL;
-
     for (uint32_t i = c->track_count; i > 0; i--) {
         av_log(s, AV_LOG_TRACE, "Compare track %d timestamp " AVRATIONAL_FORMAT
                " to minimum " AVRATIONAL_FORMAT