drm/vmwgfx: NULL pointer dereference in vmw_surface_define_ioctl()

commit 36274ab8 upstream.

Before memory allocations vmw_surface_define_ioctl() checks the
upper-bounds of a user-supplied size, but does not check if the
supplied size is 0.

Add check to avoid NULL pointer dereferences.

Signed-off-by: Murray McAllister <murray.mcallister@insomniasec.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>