bug#38587: base64-decode-region breaks encoding

[Juri Linkov]

0. emacs -Q
1. insert a non-ASCII char, e.g. ä
2. select the region around the char
3. M-x base64-encode-region
4. select the region around the encoded text
5. M-x base64-decode-region

results in a broken text.  IOW, base64-encode-region and base64-decode-region
are not reversible, whereas their string counterparts are:

(base64-decode-string (base64-encode-string "ä"))
=> "\344"

(the latter expression returns the right result, but inserts broken text too)