htdigest: Fix buffer overflow when reading digest

password file with very long lines.

PR 54893.

Backport of r1475878 from trunk resp.
r1476089 from 2.4.x.

Proposed/Backported by: rjung
Reviewed by: humbedooh, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1476242 13f79535-47bb-0310-9956-ffa450edef68