screen: ehm... no, that was wrong: page_start /is/ a column position

(The mistake becomes visible when you go to the end of a very long line
of double-width characters: the $ will be shown on the right edge, even
though you're already at line's end.)

This reverts commit 16a7fd4b from yesterday.

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 {