vpxdec: Initialize buffer variable to NULL
vpxdec: Initialize buffer variable to NULL

False positive but trivial to fix and possibly causing compiler warnings at
some point in the future too.

CID 1346535

hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good