Ignore null-pointer-arithmetic warnings in external/dlmalloc

Suppress new null-pointer-arithmetic warning introduced in next compiler
update.

Test: m checkbuild
Change-Id: I3fe71ea24813a4759881a0a443c6d24025f73b25
diff --git a/runtime/gc/allocator/dlmalloc.cc b/runtime/gc/allocator/dlmalloc.cc
index 6506220..4570e9c 100644
--- a/runtime/gc/allocator/dlmalloc.cc
+++ b/runtime/gc/allocator/dlmalloc.cc
@@ -37,6 +37,7 @@
 #pragma GCC diagnostic ignored "-Wredundant-decls"
 #pragma GCC diagnostic ignored "-Wempty-body"
 #pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#pragma GCC diagnostic ignored "-Wnull-pointer-arithmetic"
 #include "../../../external/dlmalloc/malloc.c"
 // Note: malloc.c uses a DEBUG define to drive debug code. This interferes with the DEBUG severity
 //       of libbase, so undefine it now.
diff --git a/runtime/gc/allocator/dlmalloc.h b/runtime/gc/allocator/dlmalloc.h
index 29b96ee..b12691a 100644
--- a/runtime/gc/allocator/dlmalloc.h
+++ b/runtime/gc/allocator/dlmalloc.h
@@ -32,6 +32,7 @@
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wredundant-decls"
+#pragma GCC diagnostic ignored "-Wnull-pointer-arithmetic"
 #include "../../external/dlmalloc/malloc.h"
 #pragma GCC diagnostic pop