[PATCHv7] nss_db: protect against empty mappings

[DJ Delorie]

Florian Weimer <fweimer@redhat.com> writes:
> Please also post the planned commit message.  Thanks.

I've been doing that in every patch, as $subject and comment text,
although my current $subject in git is:

    nss_db: fix endent wrt NULL mappings [BZ #24695] [BZ #24696]
   
nss_db allows for getpwent et al to be called without a set*ent,
but it only works once.  After the last get*ent a set*ent is
required to restart, because the end*ent did not properly reset
the module.  Resetting it to NULL allows for a proper restart.

If the database doesn't exist, however, end*ent erroniously called
munmap which set errno.

The test case runs "makedb" inside the testroot, so needs selinux
DSOs installed.