c++ clang: Compile -Wno-null-dereference

This is a mitigation for issue
https://github.com/Microsoft/bond/issues/536. A proper fix is still
needed.

Closes https://github.com/Microsoft/bond/pull/598

cmake/Config.cmake

@@ -149,6 +149,7 @@ cxx_add_compile_options(Clang
     --std=c++11
     -Wall
     -Werror
+    -Wno-null-dereference
     -Wno-unknown-warning-option
     -Wno-unused-local-typedefs)
 
@@ -158,6 +159,7 @@ cxx_add_compile_options(AppleClang
     --std=c++11
     -Wall
     -Werror
+    -Wno-null-dereference
     -Wno-unknown-warning-option
     -Wno-unused-local-typedefs)