xfree86: Wrap RRCrtcIsLeased and RROutputIsLeased to check for DIX structures

Before DIX structures are allocated for crtcs and outputs, we don't
want to call DIX randr code with NULL pointers. This can happen if the
driver sets video modes early in server initialization, which Nouveau
does in zaphod mode.

Cc: thellstrom@vmware.com
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=106772
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=106960


Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>