Fix ap_snprintf to do something reasonable when len == 0. When len == 0 it

now returns 0 without writing anything.  Previously it would behave like
sprintf().

Reviewed by:	Jim Jagielski, Ken Coar


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79767 13f79535-47bb-0310-9956-ffa450edef68