Fix incorrect DCHECK in generic_decoder.cc.

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 1808dc7fd19857cbae1f9e0c95ac80258d52a60f..cc14794e090ce94121bbec4af8ef7458ae61f03a 100644
--- a/webrtc/modules/video_coding/generic_decoder.cc
+++ b/webrtc/modules/video_coding/generic_decoder.cc
@@ -196,7 +196,7 @@ VCMGenericDecoder::~VCMGenericDecoder() {
   decoder_->Release();
   if (_isExternal)
     decoder_.release();
-  RTC_DCHECK(_isExternal || !decoder_);
+  RTC_DCHECK(_isExternal || decoder_);
 }
 
 int32_t VCMGenericDecoder::InitDecode(const VideoCodec* settings,