<stddef.h>: define NULL with type void *

ISO C2011 7.19 para 3 says "NULL, which expands to an implementation-defined null pointer constant"

ISO C2011 6.3.2.3 para 3 says "An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant."

So, it seems NULL can be defined either as "0" or as "(void *) 0".  However, the two definitions are not equivalent in the context of a call to a variadic or unprototyped function: passing 0 when the function expects a pointer value can go wrong if sizeof(int) != sizeof(void *).

This commit changes the definition of NULL to the safer one:

This is what GCC and Clang do in C mode; they use "#define NULL 0" for C++ only.

Fixes issue AbsInt#265

Author: xavierleroy

runtime/include/stddef.h

@@ -108,7 +108,7 @@ typedef signed int wchar_t;
 
 #if defined(_STDDEF_H) || defined(__need_NULL)
 #ifndef NULL
-#define NULL 0
+#define NULL ((void *) 0)
 #endif
 #undef __need_NULL
 #endif