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>

fs/squashfs/decompressor_multi.c

@@ -119,11 +119,10 @@ void squashfs_decompressor_destroy(struct squashfs_sb_info *msblk)
 			kfree(decomp_strm);
 			stream->avail_decomp--;
 		}
-	}
-
 		WARN_ON(stream->avail_decomp);
 		kfree(stream->comp_opts);
 		kfree(stream);
+	}
 }