(ITS#4572) back-bdb lock bug with virtual root entry

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.3.21
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (24.126.120.178)
Submitted by: hyc


For databases using the zero-length suffix, when searching for a nonexistent
entry under the root, dn2entry will return the root as the matched entry, but
without a read lock on it. The callers of dn2entry always try to unlock the
matched entry when they're finished. The BDB library can hang or crash when
trying to unlock something that was never locked.