str: Make ss_alloc_substring_pool() null-terminate its output.

It's inconsistent that ss_alloc_substring() null-terminates its output
but ss_alloc_substring_pool() does not.  This caught us out in
recode_substring_pool(), which used ss_alloc_substring_pool() in a
fallback case where create_iconv() failed and expected the result to
be null-terminated.

Reported-by: Jeremy Lavergne <jeremy@lavergne.gotdns.org>