[Issue 3363] std.stream.readf segfaults with immutable format strings

[PlatisYialos]

Indeed this issue is still open.

I've resorted to doing the following, in order to make obvious 
(to a future reader of the code) the bug:

int charsRead = fin.readf(cast(char []) "%c\n", &myChar);

Maybe a unittest should be included in the appropriate place 
within stream.d?