mod_slotmem_shm: follow up to r1702450.

r1702450 changed the behaviour of slotmem_{create,attach}() when given
given an absolute (SHM file )name.
Don't mangle the SHM file name in this case, it's up to the caller to
provide a unique name per call when this matters.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1703149 13f79535-47bb-0310-9956-ffa450edef68