Fix incorrect DCHECK in generic_decoder.cc.
When ownership is not external, the decoder pointer should be valid.

BUG=b/63658384
TBR=magjed@webrtc.org

Review-Url: https://codereview.webrtc.org/2975383002
Cr-Commit-Position: refs/heads/master@{#19041}
diff --git a/webrtc/modules/video_coding/generic_decoder.cc b/webrtc/modules/video_coding/generic_decoder.cc
index 1808dc7..cc14794 100644
--- a/webrtc/modules/video_coding/generic_decoder.cc
+++ b/webrtc/modules/video_coding/generic_decoder.cc
@@ -196,7 +196,7 @@
   decoder_->Release();
   if (_isExternal)
     decoder_.release();
-  RTC_DCHECK(_isExternal || !decoder_);
+  RTC_DCHECK(_isExternal || decoder_);
 }
 
 int32_t VCMGenericDecoder::InitDecode(const VideoCodec* settings,