`tal_strndup(ctx, NULL, 1)` breaks contract

[whitslack]

The contract for tal_strndup says:

ccan/ccan/tal/str/str.h

Lines 22 to 30 in cd56b18

	/**
	* tal_strndup - duplicate a limited amount of a string.
	* @ctx: NULL, or tal allocated object to be parent.
	* @p: the string to copy (can be take()).
	* @n: the maximum length to copy.
	*
	* Always gives a nul-terminated string, with strlen() <= @n.
	* The returned string will have tal_count() == strlen() + 1.
	*/

However, when tal_strndup(ctx, NULL, n) is called with n being any non-zero integer, the returned string will have tal_count() equal to n + 1 (i.e., at least 2), but strlen() will be equal to zero. 2 != 0 + 1.

ccan/ccan/tal/str/str.c

Lines 27 to 30 in cd56b18

	if (likely(p))
	len = strnlen(p, n);
	else
	len = n;

It seems like this^ really should say len = 0; in the else branch.