Fix no-posix-io compile failure

The fix in conf_include_test.c  seems to be required because some
compilers give an error if you give an empty string for the second
argument to strpbrk(). It doesn't really make sense to send an empty
string for this argument anyway, so make sure it has at least one character
in it.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5666)