Fix segfault in mod_alias introduced in r1132494.

AliasMatch does not append unmatched parts of the
original URI to the  new URI. So no need to subtract
anything from the new URI length.

The existing code crashed when using
"AliasMatch / /some/thing" and sending a request
with a long URI.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1583175 13f79535-47bb-0310-9956-ffa450edef68