screen: don't compare a character index with a column position

This addresses part of https://savannah.gnu.org/patch/?9216

.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

src/winio.c

@@ -2711,7 +2711,7 @@ int update_line(filestruct *fileptr, size_t index)
 #endif
 	if (page_start > 0)
 	    mvwaddch(edit, line, margin, '$');
-	if (strlenpt(fileptr->data) > page_start + editwincols)
+	if (strlenpt(fileptr->data + page_start) > editwincols)
 	    mvwaddch(edit, line, COLS - 1, '$');
 #ifndef NANO_TINY
     } else {