Just do nothing if passed NULL for a ref or unref. This avoids
call sites that manage a combination of NULL or non-NULL pointers
having to add iffery around every ref and unref.
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
8ffad850ef