Fix issue #75

There is already a check around line 2393 (global InsertPlayerFrame) which checks for:
```if Player:FindFirstChild("leaderstats") then```
In this case, it didn't work well because StatAdded() can yield (framelock etc).
During the yield the leaderstats can get removed, making the event connections error.

With this simple replacement, there should be no problem about a missing leaderstats value.

See issue #75