Stop vbprintf from core dumping if given a NULL string pointer.

Submitted by: Ken Coar <coar@decus.org>
Reviewed by: Marc Slemko, Dean Gaudet, Jim Jagielski


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@77614 13f79535-47bb-0310-9956-ffa450edef68