vp9_enc_grp_get_next_job: check job queue alloc
vp9_enc_grp_get_next_job: check job queue alloc

+ reverse conditional order; var == constant is more readable

Change-Id: I9f2b4394024c262fd5fe9576a8bf33afe197c050

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx