Fix: illegal memory access in viewer_list_sessions

Found by Coverity:

CID 1243025 (#1 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)17. buffer_size_warning: Calling strncpy with a
maximum size argument of 64 bytes on destination array
send_session->hostname of size 64 bytes might leave the destination
string unterminated.

CID 1243025 (#2 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)17. buffer_size_warning: Calling strncpy with a
maximum size argument of 255 bytes on destination array
send_session->session_name of size 255 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>