mod_substitute: Fix memory limitation in case of
regexp plus flatten.

The maxlen argument of ap_varbuf_regsub() is unsigned.
Passing in "AP_SUBST_MAX_LINE_LENGTH - vb.strlen"
in case vb.strlen got to big didn't result in the
expected error but instead was handled as a very big
maxlen.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1628104 13f79535-47bb-0310-9956-ffa450edef68