root: Do not end up in an infinite loop if the wallpaper is `nil`.

If `root.wallpaper` was called with `nil` during initialization,
AwesomeWM would get into an infinite restart loop.

root.c

@@ -481,6 +481,11 @@ luaA_root_wallpaper(lua_State *L)
 {
     if(lua_gettop(L) == 1)
     {
+        /* Avoid `error()s` down the line. If this happens during
+         * initialization, AwesomeWM can be stuck in an infinite loop */
+        if(lua_isnil(L, -1))
+            return 0;
+
         cairo_pattern_t *pattern = (cairo_pattern_t *)lua_touserdata(L, -1);
         lua_pushboolean(L, root_set_wallpaper(pattern));
         /* Don't return the wallpaper, it's too easy to get memleaks */
@@ -602,7 +607,7 @@ const struct luaL_Reg awesome_root_methods[] =
     { "cursor", luaA_root_cursor },
     { "fake_input", luaA_root_fake_input },
     { "drawins", luaA_root_drawins },
-    { "wallpaper", luaA_root_wallpaper },
+    { "_wallpaper", luaA_root_wallpaper },
     { "size", luaA_root_size },
     { "size_mm", luaA_root_size_mm },
     { "tags", luaA_root_tags },

tests/test-wallpaper.lua

@@ -133,6 +133,15 @@ end
 return true
 end)
 
+-- Make sure passing `nil` doesn't crash Awesome.
+table.insert(steps, function()
+
+root._wallpaper(nil)
+root.wallpaper(nil)
+
+return true
+end)
+
 
 runner.run_steps(steps)