wrapping: compute the width of a succeeding line in the correct manner

Its number of columns needs to be computed, not its number of bytes.

src/text.c

@@ -1466,7 +1466,7 @@ bool do_wrap(void)
 	/* We prepend the wrapped text to the next line, if the prepend_wrap
 	 * flag is set, there is a next line, and prepending would not make
 	 * the line too long. */
-	if (prepend_wrap && rest_length + strlen(line->next->data) <= wrap_at) {
+	if (prepend_wrap && rest_length + strlenpt(line->next->data) <= wrap_at) {
 		const char *tail = remainder + move_mbleft(remainder, rest_length);
 
 		/* Go to the end of the line. */