mod_disk_cache: Make sure that only positive integers are accepted

for the CacheMaxFileSize and CacheMinFileSize parameters in the
config file. PR39380

+1: minfrin, jim, trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@463506 13f79535-47bb-0310-9956-ffa450edef68