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 <[e-mail]>
Signed-off-by: Phillip Lougher <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>

fs/squashfs/decompressor_multi.c

···
 			kfree(decomp_strm);
 			stream->avail_decomp--;
 		}
+		WARN_ON(stream->avail_decomp);
+		kfree(stream->comp_opts);
+		kfree(stream);
 	}
-
-	WARN_ON(stream->avail_decomp);
-	kfree(stream->comp_opts);
-	kfree(stream);
 }