ed(1): Preserve const qualifier on strchr(3) result

Reviewed by:	fuz
Approved by:	fuz (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D59773

bin/ed/io.c

@@ -297,7 +297,7 @@ put_tty_line(const char *s, int l, long n, int gflag)
 {
 	int col = 0;
 	int lc = 0;
-	char *cp;
+	const char *cp;
 	wchar_t wc;
 	mbstate_t mbs;
 	size_t clen;