(ITS#8797) improper use of gnutls causes segfault

Full_Name: Lukas K. Version: 2.4.44+dfsg-5+deb9u1 OS: Debian Stretch URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (2001:7c7:2100:130:b9b5:f8df:7b98:8c2e)

During initialization, libldap sets custom gnutls mutex functions: https://www.openldap.org/devel/gitweb.cgi?p=openldap.git;a=blob;f=libraries/...

PAM uses libldap via dlopen and unloads it when it's done, but openldap doesn't undo gnutls_global_set_mutex, so any further calls to locking functions inside openldap will segfault since these function pointers now point to nowhere since openldap is unloaded.

I encountered this issue in cups since cups uses gnutls itself for the web interface and segfaults when it uses gnutls after libldap.