display: show the guide stripe for double-width/multi-byte characters

Determine the actual number of bytes the striped character consists of,
instead of assuming it's simply one, and determine the real column that
the character starts in, instead of assuming it's the stripe column.

This fixes https://savannah.gnu.org/bugs/?55917
and fixes https://savannah.gnu.org/bugs/?55922.

Condensed-by: Benno Schulenberg <bensberg@telfort.nl>

src/winio.c

@@ -2687,12 +2687,19 @@ void edit_draw(filestruct *fileptr, const char *converted,
 
 #ifndef NANO_TINY
 	if (stripe_column > from_col && !inhelp) {
-		const ssize_t target_column = stripe_column - from_col - 1;
-		const char *text = converted + actual_x(converted, target_column);
-		const char *striped_char = (*text == '\0') ? " " : text;
+		ssize_t target_column = stripe_column - from_col - 1;
+		size_t target_x = actual_x(converted, target_column);
+		char striped_char[MAXCHARLEN];
+		size_t charlen = 1;
+
+		if (*(converted + target_x) != '\0') {
+			charlen = parse_mbchar(converted + target_x, striped_char, NULL);
+			target_column = strnlenpt(converted, target_x);
+		} else
+			striped_char[0] = ' ';
 
 		wattron(edit, interface_color_pair[GUIDE_STRIPE]);
-		mvwaddnstr(edit, row, margin + target_column, striped_char, 1);
+		mvwaddnstr(edit, row, margin + target_column, striped_char, charlen);
 		wattroff(edit, interface_color_pair[GUIDE_STRIPE]);
 	}