* src/fileio.c: Fix bug#36431

* src/fileio.c: Fix bug#36431

(decide_coding_unwind): Re-introduce.  Move text back to the gap.
Return the new `inserted` via the unwind_data.
(Finsert_file_contents): Use it.
Make sure `inserted` is always 0 when we jump straight to `notfound`.
Don't insert the text in the buffer until we know it's properly decoded
for the byteness of the buffer.

* test/src/fileio-tests.el (fileio-tests--insert-file-interrupt):
Allow insert-file-contents to return an empty buffer in case of
non-local exit in set-auto-coding-function.