Fix heap buffer overflow in fgetwln()

Fix heap buffer overflow in fgetwln()

In the function fgetwln() there's a 4 byte heap overflow.

There is a while loop that has this check to see whether there's still
enough space in the buffer:

		if (!fb->len || wused > fb->len) {

If this is true more memory gets allocated. However this test won't be
true if wused == fb->len, but at that point wused already points out
of the buffer. Some lines later there's a write to the buffer:

		fb->wbuf[wused++] = wc;

This bug was found with the help of address sanitizer.

Warned-by: ASAN
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=93881
Signed-off-by: Guillem Jover <guillem@hadrons.org>