patch: fix sigsegv in neb_deregister_callback

[Matthew Kent]

Tiny patch to fix a sigsegv in neb_deregister_callback when only one
callback of the type exists in the list.

Also enclosed a function that I think would be handy to have in nagios
proper instead of leaving it in my module. 

As part of some buffering I'm doing in my module I'm scheduling a
recurring event to call a data dumping function to keep results fresh.
After a fatal error of some kind in the module I need to remove this
event to keep things neat and tidy.
-- 
Matthew Kent <mkent at magoazul.com>
http://magoazul.com
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