[player] Refine VideoDmpReader::video_stream_info()

It tried to return the stream info of the first video access unit, and
may crash when VideoDmpReader is working under read on demand mode.

Now `EnsureSampleLoaded(0)` is called before accessing the first video
access unit.

b/271618424

Change-Id: Ia097881daced821b0dba5df6773cbfe6abe3e34f
NOKEYCHECK=True
GitOrigin-RevId: 876612fa799700d6c2de7b842261dbd26b236246

starboard/shared/starboard/player/video_dmp_reader.h

@@ -110,7 +110,8 @@ class VideoDmpReader {
   const media::AudioStreamInfo& audio_stream_info() const {
     return dmp_info_.audio_sample_info.stream_info;
   }
-  const media::VideoStreamInfo& video_stream_info() const {
+  const media::VideoStreamInfo& video_stream_info() {
+    EnsureSampleLoaded(kSbMediaTypeVideo, 0);
     SB_DCHECK(!video_access_units_.empty());
     return video_access_units_[0].video_sample_info().stream_info;
   }