Disable warnings triggered in Clang r271374

http://b/28149048
http://b/29823425

Disable -Wnull-dereference.  The second bug above tracks that this
warning gets re-enabled.

Test: Tested build, boot and common usage for Arm, Arm64, x86, x86_64,
Mips images in AOSP and internal branch.

Change-Id: Ie41a495dd9bb247d3609f4848d7669d1faaeeb79