(ITS#3559) OpenLDAP can crash in back-hdb

[lukeh@padl.com]

Full_Name: Luke Howard
Version: 2.2.13
OS: Linux
URL: http://www.padl.com/~lukeh/dn2id.diff
Submission from: (NULL) (203.13.32.92)


In hdb_dn2idl_internal(), it is possible for the gotit branch to be executed
with cx->ei == NULL. This causes slapd to crash.

I have only been able to reproduce this on x86-64 but there is no reason why it
could not occur on other platforms.

Switching the order of the first two blocks (as done in the attached patch)
fixes this.