platform: msm_shared: check if cmdline is NULL before using it
platform: msm_shared: check if cmdline is NULL before using it

Pointer 'final_cmdline' returned from call to function
'update_cmdline' may be NULL, will be passed to function
and may be dereferenced there by passing argument 2 to
function 'update_device_tree'.

Change-Id: I6cd52dd9b7799f0ecb3fc863497647ad653a5e9d
CRs-Fixed: 687723

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