crypto: algapi - make crypto_drop_spawn() a no-op on uninitialized spawns

Make crypto_drop_spawn() do nothing when the spawn hasn't been
initialized with an algorithm yet.  This will allow simplifying error
handling in all the template ->create() functions, since on error they
will be able to just call their usual "free instance" function, rather
than having to handle dropping just the spawns that have been
initialized so far.

This does assume the spawn starts out zero-filled, but that's always the
case since instances are allocated with kzalloc().  And some other code
already assumes this anyway.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>