[PATCH] make printk more robust with "null" pointers

Expand printk's traditional handling of null pointers so that anything in the
first page is considered a null pointer.

This gives us better behaviour when someone (acpi..) accidentally prints a
string which is embedded in a struct, the pointer to which is null.

lib/vsprintf.c

@@ -348,7 +348,7 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 
 			case 's':
 				s = va_arg(args, char *);
-				if (!s)
+				if ((unsigned long)s < PAGE_SIZE)
 					s = "<NULL>";
 
 				len = strnlen(s, precision);