Groupchat crashing bug fixed.

`delpeer()` will delete the `chat->group` pointer if it sees the array is empty. Since another call to `chat->group` comes directly after, and `chat->group` was freed, then it would crash. Now it just makes a check before doing so.

This resolves https://github.com/Tox/toxic/issues/75 with the issue of crashing after computer sleeps, all peers timeout, so all get deleted in the first `delpeer()`