Squashfs: Check stream is not NULL in decompressor_multi.c

Fix static checker complaint that stream is not checked in
squashfs_decompressor_destroy().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Reviewed-by: Minchan Kim <minchan@kernel.org>