undo: make sure the current line is defined before it is referenced

After undoing an <Enter> or redoing a line join, it is likely that the
"eaten" and freed line was the current line.  In fact, goto_line_posx()
should not refer to it any more, but... accommodate for this and just
set openfile->current to a valid value before calling goto_line_posx().

This fixes https://savannah.gnu.org/bugs/?62952.

Bug existed since version 6.3, commit eea3e1f0.

src/text.c

@@ -540,6 +540,7 @@ void do_undo(void)
 		line->has_anchor |= line->next->has_anchor;
 		unlink_node(line->next);
 		renumber_from(line);
+		openfile->current = line;
 		goto_line_posx(u->head_lineno, original_x);
 		break;
 	case BACK:
@@ -746,6 +747,7 @@ void do_redo(void)
 		strcat(line->data, u->strdata);
 		unlink_node(line->next);
 		renumber_from(line);
+		openfile->current = line;
 		goto_line_posx(u->tail_lineno, u->tail_x);
 		break;
 	case REPLACE: