bug#22667: string->bytevector encoding error

[Josep Portella Florit]

'string->bytevector' throws 'encoding-error' when the encoding is
"UTF-16" and the length of the string is greater than 128.  The same
for the encoding "UTF-32" and a string of length greater than 64.

Tested on Guile 2.0.11 and 2.1.2 with the same result.

How to reproduce:

(use-modules (ice-9 iconv))

;; These expressions evaluate as expected:
(string->bytevector (make-string 128 #\A) "UTF-16")
;; => #vu8(254 255 0 65 0 65 ...)
(string->bytevector (make-string 64 #\A) "UTF-32")
;; => #vu8(0 0 254 255 0 0 0 65 0 0 0 65 ...)

;; These expressions throw 'encoding-error':
(string->bytevector (make-string 129 #\A) "UTF-16")
(string->bytevector (make-string 65 #\A) "UTF-32")