kernel/signal: prevent back reference
kernel/signal: prevent back reference

This patch prevents back references to null pointers.

Change-Id: I49adaeaeddc36c0c386aee7e48bf02b8f332c710
Signed-off-by: Junhwan Park <7fee488b6f78fc50b291ffc7d10428788977e877@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT