High: Core: Fix crash in the membership code preventing node shutdown
Due to cs: 45dda62252ee, crm_node_t entries were added to the hash table with
a NULL key, this resulted in attrd crashing during shutdown and preventing the
rest of the stack from shutting down.
Attrd was more easily affected as it doesn't subscribe to membership events
and therefor executed the 'fix' more often.

Mercurial revision: 14c9726a7e54375b0558e7ba6bc1c2a6b30efb5d