undo: do not try to paste back an empty cutbuffer

Trying to do so would dereference a NULL pointer in copy_node().

This fixes https://savannah.gnu.org/bugs/?57915.

Bug existed since before version 2.2.0.

src/text.c

@@ -762,7 +762,8 @@ void do_redo(void)
 	case INSERT:
 		redidmsg = _("insertion");
 		goto_line_posx(u->head_lineno, u->head_x);
-		copy_from_buffer(u->cutbuffer);
+		if (u->cutbuffer)
+			copy_from_buffer(u->cutbuffer);
 		free_lines(u->cutbuffer);
 		u->cutbuffer = NULL;
 		break;