compositor-drm: pass NULL to mmap() instead of 0 as the address

mmap() function expects to be passed a void pointer as the address
here. Passing NULL is technically more correct than passing 0.

Signed-off-by: Chris Michael <cp.michael@samsung.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

src/compositor-drm.c

@@ -308,7 +308,7 @@ drm_fb_create_dumb(struct drm_backend *b, unsigned width, unsigned height)
 	if (ret)
 		goto err_add_fb;
 
-	fb->map = mmap(0, fb->size, PROT_WRITE,
+	fb->map = mmap(NULL, fb->size, PROT_WRITE,
 		       MAP_SHARED, b->drm.fd, map_arg.offset);
 	if (fb->map == MAP_FAILED)
 		goto err_add_fb;