The issue was setting the static function pointer `old_handler` to
the function `ignore_underflow`, which itself calls `old_handler.
This patch is a hacky workaround. A better solution would be to
maintain a thread-local stack of GSL error handlers, perhaps using
the C11 `thread_local` access specifier.
See, for example, <http://en.cppreference.com/w/c/thread/thread_local>.
Original: c1a050cdb05f9f0d3bb749e1d012e14be87ee676