extra sanity check to ensure not too many file descriptors are open (a9aea20f136026d6553295727350c88fe2431e0c from 0.9)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.8@1935 ef36b2f9-881f-0410-afb5-c4e39611909c