Fix segmentation fault if writing to audit log fails

A segmentation fault occurs if concurrent audit log format is enabled
(SecAuditLogType Concurrent) and writing to the log fails.
Log why writing to audit log failed

Add error description from apr_strerror() to message that is logged if
writing to audit log failed.
Merge pull request #668 from edieterich/auditlog_segfault

Fix segmentation fault on auditlog