Check for valid bag slot before adding tooltip to it

[DJSchaffner]

This fixes issues with custom buttons with custom tooltips from other addons.

Example: BetterBags creates a dummy button for displaying empty bag slots.

Before:

After:

[Rottenbeer]

Sorry for the delay. Is this still relevant?

[DJSchaffner]

I am currently not actively playing SoD/Classic and have been using a fork from @romracer with this issue being fixed, so one of his open pull requests might also fix this problem. But in general this problem is probably still relevant.

[Rottenbeer]

@DJSchaffner can you rebase this please?

[DJSchaffner]

Will take a look at it tomorrow evening :)

[romracer]

My fork has #259 applied which makes this PR not relevant. Since that's been merged here, this can probably be closed.