Missing NULL check on hammer2_inode_chain in xop_inode_connect — latent dead-code NULL deref

Summary

hammer2_xops.c:1488 chain=hammer2_inode_chain(xop->head.ip2,clindex,ALWAYS). :1490 hammer2_chain_modify(chain,...) — derefs chain->hmp unconditionally NO NULL check. Every other xop checks NULL (nrename:566-571 create:1132-1138 destroy:1358-1363). LATENT DEAD CODE: hammer2_inode_connect_desc declared hammer2.h:1860 defined hammer2_admin.c:62 but NEVER passed to hammer2_xop_start anywhere in tree. If wired up in future: sparse cluster chain NULL -> hammer2_chain_modify(NULL) panic. Fix: if(chain==NULL) error=EIO goto fail.