fix thrown shield stance swapping

[gmriggs]

Thanks to kewne for reporting this issue

repro steps:

• equip a thrown weapon
• enter combat stance
• equip a shield

expected:

• thrown shield stance is transitioned into without exiting combat stance

actual:

• shield is equipped and combat stance is exited

similar problems with unequipping shield while in thrown shield combat stance, and swapping between sword+shield and thrown+shield while in combat stance also fixed