accel: don't crash on fallback

If no accel is found in getFormatCb, accel->codecCtx_ is null, and
segfaults.

Change-Id: Ibe3b246aaa0620f2dd5b91d2864fff65b6e8b794

src/media/video/accel.cpp

@@ -116,10 +116,12 @@ HardwareAccel::fail(bool forceFallback)
         fallback_ = true;
         allocationFails_ = 0;
         extractionFails_ = 0;
+        if (codecCtx_) {
             codecCtx_->get_format = avcodec_default_get_format;
             codecCtx_->get_buffer2 = avcodec_default_get_buffer2;
         }
     }
+}
 
 bool
 HardwareAccel::extractData(VideoFrame& input)