Coverity defects
    
    12811: Unused pointer value
           preop_modrdn (plugins/uiduniq/uid.c)
           Removed unused variable "dn".
    
    12812: Unused pointer value
           ldbm_back_next_search_entry_ext (slapd/back-ldbm/ldbm_search.c)
           Removed unused variable "base".
    
    12813: Uninitialized pointer read
           string_values2keys (plugins/syntaxes/string.c)
           Commit f6b74ad18a52720c4558d4a46cbb2eeb21109033 missed to set
           an value bvp for the newly added case ((syntax & SYNTAX_DN) &&
           (value_flags & SLAPI_ATTR_FLAG_NORMALIZED_CES)), which caused
           the uninitialized pointer read.  This patch sets bvp for all
           3 cases.
    (cherry picked from commit c952e16707a6a0ad98c6985484b58d0844fb8aa0)