Core/Instance: Fix GetInstanceSave sometimes checking the wrong map

[roc13x]

Changes proposed:

• Always check the correct map ID in GetInstanceSave. If the player was in a group, it would check for a save on the players current map instead of the requested mapID

Target branch(es): 3.3.5/6.x

Tests performed: Builds, works correctly ingame

[jackpoz]

thank you for the fix. do you know if this solves any known issue too ?

[ghost]

Any chance this helps with #15780 ?

[roc13x]

I don't know of any reported issues on the tracker, but it affected the "change dungeon entrance" feature on 6.x (you could no longer exit an instance through the portal)

@Nyeriah It wouldn't help with that, but I am currently testing a fix for that bug and will PR if it works