dev: gcdb: add NULL check before accessing slave panel node
dev: gcdb: add NULL check before accessing slave panel node

The slave panel node is valid only for dual DSI panels. Add a
NULL check before accessing the slave panel string.

Change-Id: I156dca8e581a3fc2bc9518e35846b53ef70658f5

Foxda-Tech/polaris-bootable-bootloader-lk,M1cha/android_bootable_bootloader_lk,efidroid/lk,CanarySolutions/little-kernel,M1cha/android_bootable_bootloader_lk,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,M1cha/lktris,M1cha/android_bootable_bootloader_lk,M1cha/lktris,CanarySolutions/little-kernel,M1cha/android_bootable_bootloader_lk,CanarySolutions/little-kernel,M1cha/lktris,M1cha/lktris,efidroid/lk,efidroid/lk,CanarySolutions/little-kernel,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,M1cha/android_bootable_bootloader_lk,M1cha/android_bootable_bootloader_lk,M1cha/lktris,Foxda-Tech/polaris-bootable-bootloader-lk,CanarySolutions/little-kernel