Fix setbuf use for VMS C

The VMS C RTL has setbuf() working for short pointers only, probably
the FILE pointer will always be in P0 (the lower 4GB).  Fortunately,
this only generates a warning about possible data loss (doesn't apply
in this case) that we can simply turn off.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5789)