Fix this.sock is undefined error.

Not sure how we get a successful shutdown after closing the socket,
but the issue was reported by altere as happening in the wild.

exec/load/binkp.js

@@ -940,6 +940,7 @@ BinkP.prototype.close = function()
 				this.sendCmd(this.command.M_EOB);
 		}
 		// Attempt a super-duper graceful shutdown to prevent RST...
+		if (this.sock !== undefined) {
 			this.sock.is_writeable = false;
 			remain = this.timeout;
 			end = time() + remain;
@@ -949,6 +950,8 @@ BinkP.prototype.close = function()
 				remain = end - time();
 			} while (remain > 0);
 			this.sock.close();
+			this.sock = undefined;
+		}
 	}
 	this.tx_queue.forEach(function(file) {
 		file.file.close();