avoid segfault when sofia tries to update the callee id at the same time as the…

avoid segfault when sofia tries to update the callee id at the same time as the outbound call is transferred

src/switch_ivr.c

@@ -1417,13 +1417,13 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_session_transfer(switch_core_session_
 			if (profile->callee_id_name) {
 				switch_channel_set_variable(channel, "pre_transfer_caller_id_name", new_profile->caller_id_name);
 				new_profile->caller_id_name = switch_core_strdup(new_profile->pool, profile->callee_id_name);
-				profile->callee_id_name = NULL;
+				profile->callee_id_name = SWITCH_BLANK_STRING;
 			}
 
 			if (profile->callee_id_number) {
 				switch_channel_set_variable(channel, "pre_transfer_caller_id_number", new_profile->caller_id_number);
 				new_profile->caller_id_number = switch_core_strdup(new_profile->pool, profile->callee_id_number);
-				profile->callee_id_number = NULL;
+				profile->callee_id_number = SWITCH_BLANK_STRING;
 			}
 		}