Problem:
When `z=` delegates to `vim.ui.select()`, the picker may change the
current window before returning. `spell_suggest()` then continues to the
cursor restoration branch with the new window and assigns `prev_cursor`,
which belongs to the original window. This can leave Normal mode with an
invalid cursor position and produce E315.
Solution:
Clean up the spell suggestion state and return immediately after handing
control to `vim.ui.select()`.
8c0bf18374