Fix missing pop

Previous commit

70ea5d552e "Add support for limiting rotation of automatic face movement dir entitys"

by sapier has broken minetest's feature to open worlds. This was due to a
missing stack pop operation.

Thanks to @oleastre for reporting this bug and suggesting the fix.

src/script/common/c_content.cpp

@@ -209,6 +209,7 @@ void read_object_properties(lua_State *L, int index,
 		if (read_color(L, -1, &color))
 			prop->nametag_color = color;
 	}
+	lua_pop(L, 1);
 
 	lua_getfield(L, -1, "automatic_face_movement_max_rotation_per_sec");
 	if (lua_isnumber(L, -1)) {