Packets/Party: fix GUID byte handling in role packets

[gonzo1247]

Changes proposed:
Fix corrupted GUID handling in CMSG_SET_ROLE and SMSG_ROLE_CHANGED_INFORM.

During the packet class conversion, one GUID byte was read/written twice while another byte was skipped:

• SetRole::Read read ChangedUnit[3] twice and never read ChangedUnit[2].
• RoleChangedInform::Write wrote ChangedUnit[6] twice and never wrote ChangedUnit[5].

As a result, role selection responses could resolve to an invalid group member GUID, causing role updates to fail.

This restores the GUID byte order used by the previous raw packet handlers.

Tests performed: (Does it build, tested in-game, etc)
build
Tested ingame:

• created a party/raid
• started a role poll
• selected Tank/Healer/Damage roles
• role selection is now applied and propagated correctly

**
This regression was introduced by commit 056248c
("Core/Packets: converted CMSG_SET_ROLE and SMSG_ROLE_CHANGED_INFORM to packet class").

During the packet class conversion, one ChangedUnit GUID byte was duplicated while another one was skipped:

• SetRole::Read reads ChangedUnit[3] twice and never reads ChangedUnit[2].
• RoleChangedInform::Write writes ChangedUnit[6] twice and never writes ChangedUnit[5].

This causes CMSG_SET_ROLE to decode a corrupted ChangedUnit GUID, so Group::SetLfgRoles cannot resolve the selected group member and the selected role is not applied.