luci-lib-jsonc: Ignore non-string-or-number keys in tables

Previously, the following caused a segmentation fault:

    json.stringify({[{}] = true})

This was caused by lua_tostring() returning NULL for anything but
strings and numbers, letting json_object_object_add crash.

This patch makes jsonc ignore all keys which have no string
representation altogether.

Signed-off-by: Jan-Philipp Litza <janphilipp@litza.de>

libs/luci-lib-jsonc/src/jsonc.c

@@ -286,8 +286,9 @@ static struct json_object * _lua_to_json(lua_State *L, int index)
 			lua_pushvalue(L, -2);
 			key = lua_tostring(L, -1);
 
-			json_object_object_add(obj, key,
-								   _lua_to_json(L, lua_gettop(L) - 1));
+			if (key)
+				json_object_object_add(obj, key,
+				                       _lua_to_json(L, lua_gettop(L) - 1));
 
 			lua_pop(L, 2);
 		}