ensure that the value asertion does not panic

If there is no routing key available then the function will
return nil, nil and release the waitgroup. The waiters will
then type assert inflight.value which is a nil interface
value which then panics. In this case we should just return
nil as the originating function did.

session.go

@@ -223,7 +223,9 @@ func (s *Session) routingKeyInfo(stmt string) (*routingKeyInfo, error) {
 			return nil, inflight.err
 		}
 
-		return inflight.value.(*routingKeyInfo), nil
+		key, _ := inflight.value.(*routingKeyInfo)
+
+		return key, nil
 	}
 
 	// create a new inflight entry while the data is created