Description of problem:
When a group contains certain incorrect multivalued memberUID values, SSSD fails to sanitize the value properly. The memberUID value should contain one username, so SSSD creates fake users using the broken memberUID as their username. This causes problems during cache indexing, for example.

Version-Release number of selected component (if applicable):
sssd-1.5.1-66

How reproducible:
always

Steps to Reproduce:
1. Create a group similar to this one:

dn: badmemberuid,ou=Groups,dc=example,dc=com
memberUid: userone,usertwo,userthree
gidNumber: 5418
cn: badmemberuid
objectClass: posixGroup
objectClass: top

2. getent group badmemberuid
3. ldbsearch -H /var/lib/sss/db/cache_example.com.ldb

Actual results:
The commas are not treated as special characters.

Expected results:
The commas are escaped with a '\' to sanitize them.

The DN should be sanitized and upgrades (which index the cache) should perform fine.