Fix: illegal memory access in _cmd_enable_event

Found by Coverity:

CID 1321742 (#1 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)21. buffer_size_warning: Calling strncpy with a
maximum size argument of 256 bytes on destination array attr->name of
size 256 bytes might leave the destination string unterminated.

CID 1321742 (#2 of 2): Buffer not null terminated
(BUFFER_SIZE_WARNING)22. buffer_size_warning: Calling strncpy with a
maximum size argument of 256 bytes on destination array attr->name of
size 256 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>