Fix: illegal memory access in relayd_create_session_2_4

Fix: illegal memory access in relayd_create_session_2_4

Found by Coverity:
CID 1243024 (#1 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)2. buffer_size_warning: Calling strncpy with a
maximum size argument of 255 bytes on destination array msg.session_name
of size 255 bytes might leave the destination string unterminated.

CID 1243024 (#2 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)3. buffer_size_warning: Calling strncpy with a
maximum size argument of 64 bytes on destination array msg.hostname of
size 64 bytes might leave the destination string unterminated.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>