WINTERMUTE: Fix MSVC Warning

This is due to the _lastLine variable being unsigned, but having
-1 assigned to it as a default / invalid value. This could probably
be assigned 0 with no issues, but since this is exposed, this warning
is fixed instead by assigning the unsigned equivalent of -1 so no
dependent code will see a change.

engines/wintermute/debugger/debugger_controller.cpp

@@ -188,7 +188,7 @@ Error DebuggerController::stepFinish() {
 
 void DebuggerController::clear() {
 	_lastScript = nullptr;
-	_lastLine = -1;
+	_lastLine = 0xFFFFFFFF; // Invalid
 }
 
 Common::String DebuggerController::readValue(const Common::String &name, Error *error) {