undo: do not try to copy a cutbuffer that is NULL

Allow the creation of an empty CUT undo item (because the result of a
filtering operation may be empty), but then don't crash when undoing
such an empty CUT.

This fixes https://savannah.gnu.org/bugs/?57929.

Bug existed since version 2.9.8, when filtering was introduced.

src/text.c

@@ -470,6 +470,7 @@ void undo_cut(undostruct *u)
 {
 	goto_line_posx(u->tail_lineno, (u->xflags & WAS_WHOLE_LINE) ? 0 : u->tail_x);
 
+	if (u->cutbuffer)
 		copy_from_buffer(u->cutbuffer);
 
 	/* If originally the last line was cut too, remove an extra magic line. */
@@ -1319,11 +1320,9 @@ void update_undo(undo_type action)
 	case ZAP:
 	case CUT_TO_EOF:
 	case CUT:
-		if (!cutbuffer)
-			die("Adding empty undo item -- please report a bug\n");
 		if (u->type == ZAP)
 			u->cutbuffer = cutbuffer;
-		else {
+		else if (cutbuffer != NULL) {
 			free_lines(u->cutbuffer);
 			u->cutbuffer = copy_buffer(cutbuffer);
 		}