util.c: ensure all TEST_CHAR loops stop at the null terminator

In the aftermath of CVE-2017-7668, decouple the business logic ("is NULL
a T_HTTP_CTRL") from the postcondition ("must not go past the end of the
string"). The NULL-byte classification in the TEST_CHAR table may change
in the future.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1799375 13f79535-47bb-0310-9956-ffa450edef68