Fix possible buffer overflow situation in suexec.c.

PS: The PR#2790 provides a few more fixes for problematic
    things in suexec.c. Because we have to be very carefully here, other
    should review them, too. That's why I commit only some of the fixes from
    this patch.

    The other problem we _really_ have to fix is the docroot-check at line
    428. But here the patch from the PR submitter seems to introduce a new
    problem: It accesses cwd[dlen] which can be out of memory bounds. Here
    memory bounds have to checked first.

    So, I would appreciate when someothers look at PR#2790, please.  At least
    the docroot-check _has_ to be fixed by us! But correctly, i.e. without
    introducing new problems, of course ;_)

Submitted by: Jeff Stewart <jws@purdue.edu>
Reviewed by: Ralf S. Engelschall
PR: 2790


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82028 13f79535-47bb-0310-9956-ffa450edef68