buffer_list_aggregate_separator(): prevent 0-byte malloc

buffer_list_aggregate_separator(): prevent 0-byte malloc

As pointed out in finding OVPN-05 of the cryptograpy engineering audit
(funded by Private Internet Access), buffer_list_aggregate_separator()
could perform a 0-byte malloc when called with a list of 0-length buffers
and a "" separator.  If other could would later try to access that buffer
memory, this would result in undefined behaviour.  To prevent this, always
malloc() 1 byte.

To simplify as we go, use alloc_buf() to allocate the buffer.  This has
the additional benefit that the actual buffer data (not the contents) is
zero-terminated, because alloc_buf() calls calloc() and we have 1 extra
byte of data.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <1514541240-19536-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16106.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>