Avoid potential segfault (found by clang/scan-build).

This is r791409 from mod_slotmem_shm.c applied to mod_slotmem_plain.c


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1001755 13f79535-47bb-0310-9956-ffa450edef68