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>