Fix pointer size issue with setbuf() on VMS

setbuf() is only for 32-bit pointers.  If compiled with /POINTER_SIZE=64,
we get a nasty warning about possible loss of data.  However, since
the only pointer used in the call is a FILE *, and the C RTL shouldn't
give us a pointer above the first 4GB, it's safe to turn off the
warning for this call.

Reviewed-by: Andy Polyakov <appro@openssl.org>