crypto: scomp - Fix null-pointer deref when freeing streams

As the scomp streams are freed when an algorithm is unregistered,
it is possible that the algorithm has never been used at all (e.g.,
an algorithm that does not have a self-test).  So test whether the
streams exist before freeing them.

Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Fixes: 3d72ad46 ("crypto: acomp - Move stream management into scomp layer")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>