gui: don't permit port in proxy IP option

Fixes: #809

Previously it was possible through the GUI to enter an IP address:port
into the "Proxy IP" configuration box. After the node was restarted the
errant setting would prevent the node starting back up until manually
removed from settings.json.

Github-Pull: bitcoin-core/gui#813
Rebased-From: 10c5275

src/qt/optionsdialog.cpp

@@ -20,6 +20,7 @@
 #include <node/chainstatemanager_args.h>
 #include <netbase.h>
 #include <txdb.h>
+#include <util/strencodings.h>
 
 #include <chrono>
 
@@ -478,7 +479,10 @@ QValidator(parent)
 QValidator::State ProxyAddressValidator::validate(QString &input, int &pos) const
 {
     Q_UNUSED(pos);
-    // Validate the proxy
+    uint16_t port{0};
+    std::string hostname;
+    if (!SplitHostPort(input.toStdString(), port, hostname) || port != 0) return QValidator::Invalid;
+
     CService serv(LookupNumeric(input.toStdString(), DEFAULT_GUI_PROXY_PORT));
     Proxy addrProxy = Proxy(serv, true);
     if (addrProxy.IsValid())