Bug 193297 - Call pre-bind plug-ins for all SASL bind steps

A pre-bind plug-in is currently only called after the final step
has been completed in a multi-step SASL bind operation.  We need
to call the pre-bind functions for each step of the bind.

ldap/servers/slapd/saslbind.c

@@ -1043,6 +1043,10 @@ void ids_sasl_check_bind(Slapi_PBlock *pb)
     case SASL_CONTINUE:         /* another step needed */
         pb->pb_conn->c_flags |= CONN_FLAG_SASL_CONTINUE;
 
+        if (plugin_call_plugins( pb, SLAPI_PLUGIN_PRE_BIND_FN ) != 0){
+            break;
+        }
+
         /* attach the sasl data */
         bvr.bv_val = (char*)sdata;
         bvr.bv_len = slen;