s3-parm: clean up defaults when removing global parameters

s3-parm: clean up defaults when removing global parameters

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11693

When globals are re-initialized, they are cleared and globals' talloc
context is freed. However, parm_table still contains a reference to the
global value in the defaults. This confuses lpcfg_string_free() after
commit 795c543d858b2452f062a02846c2f908fe4cffe4 because it tries to
free already freed pointer which is passed by lp_save_defaults():

....
    case P_STRING:
    case P_USTRING:
                  lpcfg_string_set(Globals.ctx,
                                   &parm_table[i].def.svalue,
                                   *(char **)lp_parm_ptr(NULL, &parm_table[i]));
....

here &parm_table[i].def.svalue is passed to lpcfg_string_free() but it
is a pointer to a value allocated with previous Globals.ctx which
already was freed.

This specifically affects registry backend of smb.conf in lp_load_ex()
where init_globals() called explicitly to re-init globals after
lp_save_defaults() if we have registry backend defined.

Reviewed-by: Uri Simchoni <uri@samba.org>
Signed-off-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Mon Jan 25 23:58:42 CET 2016 on sn-devel-144