Fix PAIR v0 not finalizing the pollables when closing socket (#1640)

This function needs to be pretty much identical between PAIR v0 and
v1, it was missing just the call to release the pollable resources.

src/sp/protocol/pair0/pair.c

@@ -80,6 +80,8 @@ pair0_sock_fini(void *arg)
 
  nni_lmq_fini(&s->rmq);
  nni_lmq_fini(&s->wmq);
+ nni_pollable_fini(&s->writable);
+ nni_pollable_fini(&s->readable);
  nni_mtx_fini(&s->mtx);
 }