nouveau: prevent undefined behavior in nouveau_pushbuf_reloc with gcc-4.8

Reported-by:  <ronald645@gmail.com>
Bisected-by (gcc): Andreas Radke <a.radke@arcor.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71116


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>