Fix SEGV in 'shmcb' session cache:

When a 'read' or 'write' to session cache is done, we need to check the size
of the data being 'read' or 'written' to avoid buffer over-run.

PR: 27751
Submitted by: Geoff Thorpe
Reviewed by: Madhusudan Mathihalli


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@103669 13f79535-47bb-0310-9956-ffa450edef68