notice.bro reports run-time errors when a remote connection goes away

[robin]

When a Broccoli client terminates the connection, remote.bro still accesses the list of clients with the now invalid client index. We need to add check for that.

From Craig:

1234639528.393567 run-time error: peer 10001 does not exist
1234639528.393567 /usr/local/bro/share/bro/notice.bro, line 261
(n$src_peer): internal error: field value missing

[robin]

I've attached a patch, which should fix the problem. It will still return the run-time error because that's what it is. :) Note that because we can't really return a dedicated error value, we now return the local peer val also in the case that the remote peer's connection has already terminated.

[robin]

(In [6601]) - Bugfix: Bro could crash when get_event_peer() was called after a

remote connection had already disppeared. Now it just prints a
runtime error in this case.

Closes #65.

[robin]

Patch filed in #70