[core] Fixed skip non-empty data

[gou4shi1]

Fixed a issue introduced by #2026
The skipData() after getRcvFirstMsg() can only skip empty data,
while #2026 may skip multiple non-empty ready-to-play packets that smaller than group recv base.

[gou4shi1]

Thanks for the approval, I made a typo, I'm fixing, plz don't merge yet, LOL

[gou4shi1]

Fixed, now it works fine in my production env, but the github CI failed, is it unstable or my code is buggy?

[maxsharabayko]

TestConnection.Multiple unfortunately tends to fail often. Nothing to do with your PR.