otherwise the data is just dropped. Check for current position equal to
the buffer base at the entry of the function; if not equal, setvbuf()
was done from the write method and it is not our business to override
the decision.
PR: 76398
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34007