libs: decoder: h264: remove unrequired NULL check
libs: decoder: h264: remove unrequired NULL check

Coverity scan bug:

Dereference after null check: Either the check against null is
unnecessary, or there may be a null pointer dereference.

In the original commit for fill_picture_gaps() (commit 5abd2b90) the
prev_picture could be NULL, that's why the code did a null check. But,
since commit 52adebe7, the previous reference frames are tracked, thus
there is no need to check null anymore.

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