mod_rewrite: Fix 0 bytes write into random memory position.

PR:	31036
Obtained from:
Submitted by:	nd
Reviewed by:	nd, trawick, jerenkrantz, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105431 13f79535-47bb-0310-9956-ffa450edef68