execute: don't crash when an empty buffer is piped through a command

That is, take into account that the cutbuffer could be NULL
(when updating the undo item).

This fixes https://savannah.gnu.org/bugs/?62107.

Bug existed since version 4.9, commit b15c5a7e.

src/text.c

@@ -1177,7 +1177,8 @@ void update_undo(undo_type action)
 		else if (cutbuffer != NULL) {
 			free_lines(u->cutbuffer);
 			u->cutbuffer = copy_buffer(cutbuffer);
-		}
+		} else
+			break;
 		if (!(u->xflags & MARK_WAS_SET)) {
 			linestruct *bottomline = u->cutbuffer;
 			size_t count = 0;