Don't throw fatal errors from create_temp_file()

Don't throw fatal errors from create_temp_file()

This function is called in response to connecting clients, and can fail
when I/O fails for some (possibly temporary) reason.  In such cases we
should not exit the process, but just reject the connecting client.

This commit changes the function to actually return NULL on errors, and
(where needed) changes the callers to check for and handle errors.

Since the tls-crypt-v2 metadata code also calls create_temp_file() when
clients connect, I consider this a prerequisite for tls-crypt-v2.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20171101220342.14648-4-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15701.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>