+ POS is returned. No range checking.
+
+ WARNING: The character returned by this macro could be "unified"
+ inside STRING_CHAR, if the original character in the buffer belongs
+ to one of the Private Use Areas (PUAs) of codepoints that Emacs
+ uses to support non-unified CJK characters. If that happens,
+ CHAR_BYTES will return a value that is different from the length of
+ the original multibyte sequence stored in the buffer. Therefore,
+ do _not_ use FETCH_MULTIBYTE_CHAR if you need to advance through
+ the buffer to the next character after fetching this one. Instead,
+ use either FETCH_CHAR_ADVANCE or STRING_CHAR_AND_LENGTH. */