input/decoder: return-statement with expression is ill-formed

Having written too much C++ in my days, I wrongfully assumed that it
was legal to have a return-statement with an expression in a function
returning void, as long as the expression would yield void, in C (as
it is in C++).

However, according to the C99 ISO Standard (section 6.8.6.4p1) this is
not the case.

> [ :: 6.8.6.4p1 :: ]
>
> A return statement with an expression shall not appear in a function
> whose return type is void. A return statement without an expression
> shall only appear in a function whose return type is void.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>