crypto: scomp - Fix wild memory accesses in scomp_free_streams

In order to use scomp_free_streams to free the partially allocted
streams in the allocation error path, move the alg->stream assignment
to the beginning.  Also check for error pointers in scomp_free_streams
before freeing the ctx.

Finally set alg->stream to NULL to not break subsequent attempts
to allocate the streams.

Fixes: 3d72ad46 ("crypto: acomp - Move stream management into scomp layer")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Co-developed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Co-developed-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>