(ITS#8011) few mistakes in lmdb-backend

[leo]

Full_Name: Leonid Yuriev
Version: master
OS: RHEL7
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (31.130.36.33)


Please review the patch, which will be attached shortly.

mdb_idscopes():
  + assertion for mdb_id2l_insert() result;
  - unnecessary assignment;
    
static search_aliases():
  - unused variable 'first';
  + range-check for mdb_idl_search() result;

static search_stack():
  * fix sctmp/stack allocation size;