Windows: Fix saving of custom keys (#1313)

The registry compiler couldn't handle strings beginning with a separator, which was used for marking keys defined by scan code.

src/platform/StdRegistry.cpp

@@ -498,6 +498,8 @@ bool StdCompilerConfigRead::FollowName(const char *szName)
 
 bool StdCompilerConfigRead::Separator(Sep eSep)
 {
+	// ensure string is loaded in case value begins with a separator
+	if (!LastString.getData()) LastString.Take(ReadString());
 	if (LastString.getData())
 	{
 		// separator within string: check if it is there