nasmlib.h: add a nasm_zero() macro to the pointer helpers

We do have a number of places where we had problems doing things like:

memset(foo, 0, sizeof foo) instead of sizeof *foo.  Add a helper macro
nasm_zero() to the list of (sadly not yet well used) pointer-safe
helper macros.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

include/nasmlib.h

@@ -156,6 +156,7 @@ char * safe_alloc nasm_strndup(const char *, size_t);
 #define nasm_new(p) ((p) = nasm_zalloc(sizeof(*(p))))
 #define nasm_newn(p,n) ((p) = nasm_calloc(sizeof(*(p)),(n)))
 #define nasm_delete(p) do { nasm_free(p); (p) = NULL; } while (0)
+#define nasm_zero(p) (memset((p), 0, sizeof(*(p))))
 
 /*
  * Wrapper around fwrite() which fatal-errors on output failure.