Omit XLI (init) == 0 optimization in make-vector

Omit XLI (init) == 0 optimization in make-vector

* src/alloc.c (Fmake_vector): Simplify by omitting the (XLI (init)
== 0) case, as this optimization is probably not worth the hassle.
Just for the record, the test for that case could have been
(XLI (init) % ((EMACS_UINT) -1 / UCHAR_MAX) == 0) (!),
assuming the typical platform with no padding bits and where
conversion to int omits the most significant bits.