v4l2codecs: vp9: Add missing error checks in decide_allocation
The GstV4l2Request now holds a reference on the picture buffer and is recounted already. This effectively removes usage of GRefCount which is only vailable in GLib 2.58, while we support 2.56.
p.s. also add missing error handling in the decide allocation, which leads to crash if we are running out of memory.
Fixes #910 (closed)