ulogd: ignore private data on plugin stop

ulogd: ignore private data on plugin stop

When deciding whether to call the stop hook of a plugin instance, only
two things are relevant: If the plugin actually has a stop hook defined,
and if the plugin instance is still used in a different stack. The
private data of a plugin instance is opaque to ulogd, so its size or
content are irrelevant to the stop-hook decision. And in the same vein
should ulogd never write to it.

The one-null-byte write could previously lead to an out-of-bounds write
on plugins with a stop hook and zero-size private data.

Signed-off-by: Corubba Smith <corubba@gmx.de>
Signed-off-by: Florian Westphal <fw@strlen.de>