tools/nolibc/stdio: make printf(%s) accept NULL

It's often convenient to support this, especially in test programs where
a NULL may correspond to an allocation error or a non-existing value.
Let's make printf("%s") support being passed a NULL. In this case it
prints "(null)" like glibc's printf().
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/stdio.h

@@ -220,6 +220,8 @@ int vfprintf(FILE *stream, const char *fmt, va_list args)
 			}
 			else if (c == 's') {
 				outstr = va_arg(args, char *);
+				if (!outstr)
+					outstr="(null)";
 			}
 			else if (c == '%') {
 				/* queue it verbatim */