drm/qxl: fix use of uninitialized variable

In function qxl_release_alloc(), when kmalloc() returns a NULL pointer,
it returns value 0 and parameter *ret is uninitialized. 0 means no error
to the callers of qxl_release_alloc(). The callers keep going and will
try to reference the uninitialized variable. This patch fixes the bug,
returning "-ENOMEM" when kmalloc() fails.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188911

Signed-off-by: Pan Bian <[e-mail]>
[seanpaul fixed up subject prefix]
Signed-off-by: Sean Paul <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/qxl/qxl_release.c

···
 	release = kmalloc(size, GFP_KERNEL);
 	if (!release) {
 		DRM_ERROR("Out of memory\n");
-		return 0;
+		return -ENOMEM;
 	}
 	release->base.ops = NULL;
 	release->type = type;