Fix off-by-one bugs in video_coding::PacketBuffer when the buffer is filled with a single frame.

BUG=webrtc:8028

Review-Url: https://codereview.webrtc.org/2993513002
Cr-Commit-Position: refs/heads/master@{#19209}
diff --git a/webrtc/modules/video_coding/frame_object.cc b/webrtc/modules/video_coding/frame_object.cc
index 4c4537c..663ea6f 100644
--- a/webrtc/modules/video_coding/frame_object.cc
+++ b/webrtc/modules/video_coding/frame_object.cc
@@ -93,7 +93,7 @@
     frame_type_ = first_packet->frameType;
   }
 
-  GetBitstream(_buffer);
+  RTC_DCHECK(GetBitstream(_buffer));
   _encodedWidth = first_packet->width;
   _encodedHeight = first_packet->height;