laststring is derefed even before it is overwritten

[LumioseSil]

Bug report

Bug description:

It shouldbe dereferenced after the write is done, not before.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-142433: Move deref to below the error when checking for laststring #142402
• [3.14] gh-142433: Move deref to below the error when checking for laststring (GH-142402) #142524