Only remove context from hash if there is a client id.

This should never happen, but just in case.

src/loop.c

@@ -638,9 +638,11 @@ void do_disconnect(struct mosquitto_db *db, struct mosquitto *context)
 			context->sock = INVALID_SOCKET;
 			context->pollfd_index = -1;
 		}
+		if(context->id){
 			HASH_DELETE(hh_id, db->contexts_by_id, context);
 			context->old_id = context->id;
 			context->id = NULL;
+		}
 	}else
 #endif
 	{