wsrep should not call InnoDB/XtraDB functions that are not present

E.g:

#1 0x0000555555bc9e08 in get_SE_checkpoint (unused=0x0, plugin=0x7fffffffd258, arg=0x7fffffffd310) at /home/jan/server/sql/wsrep_mysqld.cc:186
(gdb) p hton->wsrep_get_checkpoint
$1 = (int (handlerton *, XID *)) 0x0

Instead causing signal 11, we should do:

if (hton->wsrep_get_checkpoint) {

  hton->wsrep_get_checkpoint(...);

} else {

  fprintf(stderr, "BIG FAT ERROR MESSAGE\n");

  stop;

}