Correct function signature of shortuniv_assign (Closes: #983989)

Correct function signature of shortuniv_assign (Closes: #983989)

This fixes a FTBFS with gcc-11; the first argument of shortuniv_assign
is meant to be Short_Univ_int (and, indeed, that's the type that was
being used where it is called), but it was wrongly declared as
Univ_int.