bug#33749: 26.1; input-decode-map to empty vector should preserve echo area

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Date: Tue, 25 Dec 2018 14:35:13 -0500
> Cc: 33749@debbugs.gnu.org
> 
> I think The Right Thing to do is likely to move this code to
> read_key_sequence, more specifically, move it to the point where we
> *know* we really do have an event.
> 
> IOW, I think the patch below might be a better option (where we test
> `indec.start > 0` to make sure some *decoded* event was read).

Where in that patch is the variable that I asked for, which would
trigger this special behavior?