patch 8.1.0053: first argument of 'completefunc' has inconsistent type

patch 8.1.0053: first argument of 'completefunc' has inconsistent type

Problem:    The first argument given to 'completefunc' can be Number or
            String, depending on the value.
Solution:   Avoid guessing the type of an argument, use typval_T in the
            callers of call_vim_function(). (Ozaki Kiichi, closes #2993)