ring: use NULL instead of 0 for null pointers
ring: use NULL instead of 0 for null pointers

This is consistent with the rest of the code making clear
fields are pointers.

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>
Acked-by: Uri Lublin <0332784c07b6f27b46541214503bc0da50f0f291@redhat.com>
Acked-by: Victor Toso <39b7599ff73bbf5db531748ba5250f21fa7120ff@redhat.com>

fgouget/spice-common,SPICE/spice-common,SPICE/spice-common,SPICE/spice-common,fgouget/spice-common,fgouget/spice-common,fgouget/spice-common