Fix a possible bug

mirai-core-qqandroid/src/commonMain/kotlin/net/mamoe/mirai/qqandroid/network/protocol/packet/chat/NewContact.kt

@@ -136,7 +136,8 @@ internal class NewContact {
 
                 return if (struct == null) null else {
                     struct.msg?.run<Structmsg.SystemMsg, Packet> {
-                        if (c2cInviteJoinGroupFlag == 1) {
+                        when (c2cInviteJoinGroupFlag) {
+                            1 -> {
                                 // 被邀请入群
                                 BotInvitedJoinGroupRequestEvent(
                                     bot,
@@ -146,7 +147,8 @@ internal class NewContact {
                                     groupName,
                                     actionUinNick
                                 )
-                        } else {
+                            }
+                            0 -> {
                                 // 成员申请入群
                                 MemberJoinRequestEvent(
                                     bot,
@@ -158,6 +160,8 @@ internal class NewContact {
                                     reqUinNick
                                 )
                             }
+                            else -> throw IllegalStateException("Unknown c2cInviteJoinGroupFlag: $c2cInviteJoinGroupFlag in SystemMsgNewGroup packet")
+                        }
                     } as Packet // 没有 as Packet 垃圾 kotlin 会把类型推断为Any
                 }
             }