selftests/nolibc: prevent out of bounds access in expect_vfprintf

If read() fails and returns -1 (or returns garbage for some other
reason) buf would be accessed out of bounds.
Only use the return value of read() after it has been validated.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>