htdigest: Fix buffer overflow when reading digest

password file with very long lines.

PR 54893.

Backport of r1475878 from trunk.

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


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1476089 13f79535-47bb-0310-9956-ffa450edef68