Fix access to single-byte characters in buffer text

Fix access to single-byte characters in buffer text

* src/xdisp.c (get_visually_first_element)
(Fbidi_find_overridden_directionality):
* src/cmds.c (Fend_of_line): Use FETCH_BYTE instead of FETCH_CHAR,
and byte position instead of character position, to access
individual bytes of buffer text.  This avoids producing invalid
characters and accessing wrong buffer positions.  (Bug#41520)