Fix flaky test TestFlexfecRtpStatePreservation.

Videosendstream can be created before capturer starts, so initially the frame resolution may be zero. Add a check to prevent test failure and undesired behavior.

Bug: webrtc:7737
Change-Id: I8f4402e866f45ea1eb112437f866170691a111f6
Reviewed-on: https://webrtc-review.googlesource.com/95102
Commit-Queue: Ying Wang <yinwa@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24404}

modules/video_coding/media_opt_util.cc

@@ -505,8 +505,8 @@ VCMLossProtectionLogic::VCMLossProtectionLogic(int64_t nowMs)
       _shortMaxLossPr255(0),
       _packetsPerFrame(0.9999f),
       _packetsPerFrameKey(0.9999f),
-      _codecWidth(0),
-      _codecHeight(0),
+      _codecWidth(704),
+      _codecHeight(576),
       _numLayers(1) {
   Reset(nowMs);
 }