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

core/config.mk

@@ -645,8 +645,10 @@ GLOBAL_CFLAGS_NO_OVERRIDE := \
 
 GLOBAL_CLANG_CFLAGS_NO_OVERRIDE := \
     -Werror=address-of-temporary \
-    -Werror=null-dereference \
     -Werror=return-type \
+    # Bug: http://b/29823425 Disable -Wnull-dereference until the new cases
+    # detected by this warning in Clang r271374 are fixed.
+    #-Werror=null-dereference \
 
 GLOBAL_CPPFLAGS_NO_OVERRIDE :=