Avoid compiler "use of uninitialised variable" warnings

The compiler isn't smart enough to figure all of these out, so
restructure things a bit to avoid warnings.
diff --git a/vncviewer/UserDialog.cxx b/vncviewer/UserDialog.cxx
index 640f2a9..c3aa2f3 100644
--- a/vncviewer/UserDialog.cxx
+++ b/vncviewer/UserDialog.cxx
@@ -135,6 +135,12 @@
     y += 20 + 5;
     username = new Fl_Input(70, y, win->w()-70-10, 25);
     y += 25 + 5;
+  } else {
+    /*
+     * Compiler is not bright enough to understand that
+     * username won't be used further down...
+     */
+    username = NULL;
   }
 
   (new Fl_Box(70, y, win->w()-70-10, 20, _("Password:")))