Core/Channel: Fix CRT Debug Assert triggered with invalid channel names

(cherry picked from commit f0bd90cc35098475b20e736cf98885c99c04be3d)

src/server/game/Handlers/ChannelHandler.cpp

@@ -47,7 +47,7 @@ void WorldSession::HandleJoinChannel(WorldPackets::Channel::JoinChannel& packet)
     if (packet.ChannelName.empty())
         return;
 
-    if (isdigit(packet.ChannelName[0]))
+    if (isdigit((unsigned char)packet.ChannelName[0]))
         return;
 
     if (ChannelMgr* cMgr = ChannelMgr::ForTeam(GetPlayer()->GetTeam()))

src/server/scripts/Commands/cs_modify.cpp

@@ -702,7 +702,7 @@ public:
 
         amount = atoi(rankTxt);
         // try to find rank by name
-        if ((amount == 0) && (rankTxt[0] != '-') && !isdigit(rankTxt[0]))
+        if ((amount == 0) && (rankTxt[0] != '-') && !isdigit((unsigned char)rankTxt[0]))
         {
             std::string rankStr = rankTxt;
             std::wstring wrankStr;