PyUnicode_Join(): Recent code changes introduced new

PyUnicode_Join():  Recent code changes introduced new
compiler warnings on Windows (signed vs unsigned mismatch
in comparisons).  Cleaned that up by switching more locals
to Py_ssize_t.  Simplified overflow checking (it can _be_
simpler because while these things are declared as
Py_ssize_t, then should in fact never be negative).