apr: Security fix for CVE-2021-35940

An out-of-bounds array read in the apr_time_exp*() functions was fixed in the
Apache Portable Runtime 1.6.3 release (CVE-2017-12613). The fix for this issue
was not carried forward to the APR 1.7.x branch, and hence version 1.7.0 regressed
compared to 1.6.3 and is vulnerable to the same issue.

Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>