Make buffer related function conversion explicit when narrowing

Make buffer related function conversion explicit when narrowing

Clang and gcc do report many of the narrowing conversion that MSVC
reports, like these:

 warning C4267: 'function': conversion from 'size_t' to 'int', possible
loss of data

This commit changes int to size_t where it is safe
(e.g. checked by buf_size_valid) and add casts where necessary.

In the function buffer_read_from_file the return value of fread is
size_t (at least on Linux/Windows/macOS and cppreference), so fix the
check to actually make sense.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210324222330.455-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21805.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>