Add DCHECK in LibvpxVp8Encoder

Ensure that frame_buffer_controller_ does not get assigned null
by the factory.

Bug: None
Change-Id: I84e141ae0390cd024863f88cdcdc79b8b13e7c64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137043
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27962}
diff --git a/modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc b/modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc
index 0bb08c7..c29832d 100644
--- a/modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc
+++ b/modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc
@@ -490,6 +490,7 @@
     Vp8TemporalLayersFactory factory;
     frame_buffer_controller_ = factory.Create(*inst);
   }
+  RTC_DCHECK(frame_buffer_controller_);
 
   number_of_cores_ = number_of_cores;
   timestamp_ = 0;