vaapiencode: h264: remove spurious code
vaapiencode: h264: remove spurious code

Coverity scan bug:

An unsigned value can never be negative, so this test will always
evaluate the same way.

As len is guint32, there is no need to check it if it is equal or
bigger than zero.

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi