We don't return rates to randr < 1.1 clients, so don't allocate space
for them.  This fixes a FatalError due to not all allocated space being
used.

X.Org bug#21861 <http://bugs.freedesktop.org/show_bug.cgi?id=21861>
Reported-by: Guillaume Quintin <coincoin169g@gmail.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>

Backported from Arctica GH 3.6.x branch.

v2: backport to nx-libs 3.6.x (Ulrich Sibiller)
v3: backport to nx-libs 3.5.0.x (Mihai Moldovan)