Temp fix for a crash faced by client using proxy connection

[maratal]

Probably closes https://github.com/ably/ably-cocoa/issues/1437

Explanation: ARTSRProxyConnect object can potentially generate a proxy connection error even after initial success (or more than one error in a raw), which can depend on what proxy type is used, which will create a race condition: self->_proxyConnect = nil; deallocates ARTSRProxyConnect object on a _workQueue, and a subsequent call to _failWithError: on a io.ably.socketrocket.NetworkThread inside ARTSRProxyConnect will cause crash when it'll try to access self.