Don't try to quit multiple times (#1392)

Calls to CIRCSock::Quit() eventually result in the object's destructor
being called, which itself calls CIRCSock::Quit() again. Avoid sending
multiple quit messages to the remote server by checking if the
underlying socket is already marked for closing.

src/IRCSock.cpp

@@ -137,6 +137,9 @@ CIRCSock::~CIRCSock() {
 }
 
 void CIRCSock::Quit(const CString& sQuitMsg) {
+    if (IsClosed()) {
+        return;
+    }
     if (!m_bAuthed) {
         Close(CLT_NOW);
         return;