Fix: illegal memory access in cmd_snapshot_list_outputs

Found by Coverity:

CID 1243031 (#1 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)22. buffer_size_warning: Calling strncpy with a
maximum size argument of 4096 bytes on destination array (list +
idx).ctrl_url of size 4096 bytes might leave the destination string
unterminated.

CID 1243031 (#2 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)26. buffer_size_warning: Calling strncpy with a
maximum size argument of 255 bytes on destination array (list +
idx).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>