quic_family_tmpl: Fix "FORWARD_NULL" caught by coverity
quic_family_tmpl: Fix "FORWARD_NULL" caught by coverity

Ensure the received bucket is non NULL

Fantu/spice-common,SPICE/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,elmarco/spice-common,Fantu/spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common,fgouget/spice-common,elmarco/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,elmarco/spice-common,fgouget/spice-common,fgouget/spice-common,SPICE/spice-common