bug#19875: 25.0.50; Setting an input-method-function breaks prefix show on the echo area

[Eli Zaretskii]

> From: oscarfv@telefonica.net (Óscar Fuentes)
> Date: Sun, 15 Feb 2015 18:04:16 +0100
> 
> 
> emacs -Q
> M-x set-input-method ENTER spanish-prefix ENTER
> 
> Now let's suppose that we want to execute a command with a numeric
> prefix (say 42). Let's press C-u and wait until it is displayed on the
> echo area, then press `4' and wait, then press `2' and wait until it
> is displayed too. The echo area shows:
> 
> C-u C-u 4 C-u 4 2

This happens on the release branch, and in Emacs 24.4 as well.