mod_md: change types of fields of ocsp_summary_ctx_t

The number of members in ostat_by_id may be up to UINT_MAX
and there are no guarantees that all types of members (good,
revoked or unknown) are present. An integer overflow may also
occur in md_ocsp_get_summary() when they are summed as ints.

Change types of good, revoked and unknown to unsigned.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Submitted by: Anastasia Belova <nabelova31 gmail.com>
Github: closes #534


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1930710 13f79535-47bb-0310-9956-ffa450edef68