fix(skins): add safety check for uninitialized bags db in guild bank

[Xurkon]

Description

This PR adds a safety check for the Bags module database (B.db) in the Guild Bank skin.

The issue

In some scenarios (likely due to the Guild Bank UI loading early on Ascension), the GuildBankFrame_Update hook fires before the ElvUI Bags module has fully initialized its database. This lead to a Lua error: ...GuildBank.lua:129: attempt to index field 'db' (a nil value).

The Fix

• Wrapped the logic that depends on B.db (such as showBindType check and UpdateSlotAppearance) in a conditional if B.db then ... end.
• This prevents the UI from crashing during initial load while ensuring skinning functionality resumes once the module is ready.