slotmem_shm: Error detection

trunk patch: https://svn.apache.org/viewvc?view=revision&revision=1540161
             https://svn.apache.org/viewvc?view=revision&revision=1540163
             https://svn.apache.org/viewvc?view=revision&revision=1540178
             https://svn.apache.org/viewvc?view=revision&revision=1540179
             https://svn.apache.org/viewvc?view=revision&revision=1540220
             http://svn.apache.org/viewvc?view=revision&revision=r1542413
2.4.x patch: http://people.apache.org/~jim/patches/slotmem-error2.patch

Submitted by: jim
Reviewed by: jorton, minfrin



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1542803 13f79535-47bb-0310-9956-ffa450edef68