Fix: illegal memory access in cmd_snapshot_record

Found by Coverity:
CID 1243027 (#1 of 1): Buffer not null terminated
(BUFFER_SIZE_WARNING)20. buffer_size_warning: Calling strncpy with a
maximum size argument of 255 bytes on destination array tmp_output.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>