Ignore close callback when closed.
Ignore close callback when closed.

This fixes a bug where closing a bullet
connection in poll mode will not work.

studzien/bullet,extend/bullet,timCF/bullet,stwind/bullet,studzien/bullet,fenollp/bullet,ninenines/bullet,bosky101/bullet,fenollp/bullet,K2InformaticsGmbH/bullet,K2InformaticsGmbH/bullet,emb/bullet,jamhed/bullet,bosky101/bullet,emb/bullet,bearyinnovative/bullet,jamhed/bullet,lpgauth/bullet,marianoguerra/bullet,lpgauth/bullet