Fixed heap-buffer-overflow in Buffer.prototype.indexOf().

Fixed heap-buffer-overflow in Buffer.prototype.indexOf().

Previously, when `from` argument was provided heap-buffer-overflow might
happen due to lack of boundary check. `to = njs_min(to, length)`
statement was also removed because it has no effect, `to` is
equal to `length` here.

The issue was introduced in 5d15a8d6 (0.8.5).

This closes #794 issue on Github.