Allow underscores in the Windows installation path. Fixes #5467

pkg/win32/installer.iss.in

@@ -161,7 +161,7 @@ begin
     for I := 3 to Length(Path) do
       begin
         case Path[I] of
-          '0'..'9', 'A'..'Z', '\', ' ', '.', '-', '(', ')':
+          '0'..'9', 'A'..'Z', '\', ' ', '.', '-', '_', '(', ')':
           else
           begin
             Ret := False;