bug#56675: UTF16 encoding adds BOM before every single character

With this code:

(let ((p (open-output-file "x.txt")))
    (set-port-encoding! p "UTF16")
    (display "ABC" p)
    (close-port p))

the sequence of bytes in the output file x.txt is

['FF', 'FE', '41', '0', 'FF', 'FE', '42', '0', 'FF', 'FE', '43', '0']


As you can see, a BOM is being added before every
single character instead of just at the beginning.

Originally reported at
https://lists.gnu.org/archive/html/guile-user/2022-07/msg00041.html