coverity 12563 Read from pointer after free

In dna_be_txn_pre_op(): Reads target of a freed pointer
set types_to_generate to NULL after slapi_ch_array_free
Reviewed by: nkinder, nhosoi (Thanks!)

ldap/servers/plugins/dna/dna.c

@@ -3137,6 +3137,7 @@ static int dna_pre_op(Slapi_PBlock * pb, int modtype)
                     /* free up */
                     slapi_ch_free_string(&value);
                     slapi_ch_array_free(types_to_generate);
+                    types_to_generate = NULL;
                 } else if (types_to_generate) {
                     slapi_ch_free((void **)&types_to_generate);
                 }