Bug 999634 - ns-slapd crash due to bogus DN

https://bugzilla.redhat.com/show_bug.cgi?id=999634
Reviewed by: ???
Branch: 389-ds-base-1.2.11
Fix Description: When the target DN is not a valid DN, the code will bypass
the initialization of unhashed_pw_smod, and attempt to call slapi_smods_done.
Depending on what memory is in the unhashed_pw_smod, if both mods and
free_mods are true, an attempt will be made to free mods or *mods and the
server will crash.  It is tricky to find the right sequence of operations
that will write the stack in such a way that both unhashed_pw_smod.mods
and unhashed_pw_smod.free_mods are set.
The fix is to just get rid of unhashed_pw_smod which is not used.
I also check this code and other operation code for similar cases, but the
rest of the code is clean.
Platforms tested: RHEL6 x86_64
Flag Day: no
Doc impact: no
(cherry picked from commit a3d65ac00df871675896f587b0da2c24eab961bb)