Referential integrity plugin - in referint_thread_func does not handle null from ldap_utf8strtok_r

[iamarty3z]

Issue Description
Referential integrity plugin in the case when processing a line from the logfile. When a null pointer is dereferenced server crash.

Steps to Reproduce
Steps to reproduce the behavior:

1. enable the plugin using transactions (betxnpostoperation)
2. set delay > =1
3. move entry from containerscope
4. server crash

Expected results
Skipping a line in the log

Additional context
When processing the next line GetNextLine(thisline, MAX_LINE, prfd)
the ldap_utf8strtok_r function may return null.

When passing null to strcasecmp, a crash may occur inside.
The first call ldap_utf8strtok_r(NULL and the ptoken assignment are checked for null, the second and third are not.

There is a solution to this PR:
#6168