Fix a pair of potential buffer overflows in htdigest

Obtained from: Martin Schulze <joey@infodrom.org>
Reviewed by: William Rowe, Sander Striker, David Reid


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@99659 13f79535-47bb-0310-9956-ffa450edef68