crypto: ghash - Avoid null pointer dereference if no key is set

The ghash_update function passes a pointer to gf128mul_4k_lle which will
be NULL if ghash_setkey is not called or if the most recent call to
ghash_setkey failed to allocate memory.  This causes an oops.  Fix this
up by returning an error code in the null case.

This is trivially triggered from unprivileged userspace through the
AF_ALG interface by simply writing to the socket without setting a key.

The ghash_final function has a similar issue, but triggering it requires
a memory allocation failure in ghash_setkey _after_ at least one
successful call to ghash_update.

  BUG: unable to handle kernel NULL pointer dereference at 00000670
  IP: [<d88c92d4>] gf128mul_4k_lle+0x23/0x60 [gf128mul]
  *pde = 00000000
  Oops: 0000 [#1] PREEMPT SMP
  Modules linked in: ghash_generic gf128mul algif_hash af_alg nfs lockd nfs_acl sunrpc bridge ipv6 stp llc

  Pid: 1502, comm: hashatron Tainted: G        W   3.1.0-rc9-00085-ge9308cf #32 Bochs Bochs
  EIP: 00...