add null check to handshake

```
TypeError: Cannot use 'in' operator to search for 'issued' in null
at Manager.garbageCollection (node_modules/socket.io/lib/manager.js:1023:21)
```

```
handshake = this.handshaken[ids[i]];
```
to be null.

# Please enter the commit message for your changes. Lines starting
# with '#' will be kept; you may remove them yourself if you want to.
# An empty message aborts the commit.
# On branch 0.9
# Your branch is up-to-date with 'origin/0.9'.
#
# Changes to be committed:
#	modified:   lib/manager.js
#