Change DCHECK in VCMDecodedFrameCallback back to just logging.

webrtc/modules/video_coding/generic_decoder.cc

Index: webrtc/modules/video_coding/generic_decoder.cc
diff --git a/webrtc/modules/video_coding/generic_decoder.cc b/webrtc/modules/video_coding/generic_decoder.cc
index 48bf237996a9e6501405f3bad8cd30b716718493..b95571b8f73793718d318eedc82fff249e9a9f17 100644
--- a/webrtc/modules/video_coding/generic_decoder.cc
+++ b/webrtc/modules/video_coding/generic_decoder.cc
@@ -59,7 +59,6 @@ int32_t VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage,
     frameInfo = _timestampMap.Pop(decodedImage.timestamp());
     callback = _receiveCallback;
   }
-  RTC_DCHECK(callback != nullptr);
 
   if (frameInfo == NULL) {
     LOG(LS_WARNING) << "Too many frames backed up in the decoder, dropping "
@@ -77,7 +76,13 @@ int32_t VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage,
 
   decodedImage.set_render_time_ms(frameInfo->renderTimeMs);
   decodedImage.set_rotation(frameInfo->rotation);
-  callback->FrameToRender(decodedImage);
+  // TODO(sakal): Investigate why callback is NULL sometimes and replace if
+  // statement with a DCHECK.
+  if (callback) {
+    callback->FrameToRender(decodedImage);
+  } else {
+    LOG(LS_WARNING) << "No callback, dropping frame.";
+  }
   return WEBRTC_VIDEO_CODEC_OK;
 }