squashfs: more metadata hardening

Anatoly reports another squashfs fuzzing issue, where the decompression
parameters themselves are in a compressed block.

This causes squashfs_read_data() to be called in order to read the
decompression options before the decompression stream having been set
up, making squashfs go sideways.
Reported-by: NAnatoly Trosinenko <anatoly.trosinenko@gmail.com>
Acked-by: NPhillip Lougher <phillip.lougher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/squashfs/block.c

@@ -167,6 +167,8 @@ int squashfs_read_data(struct super_block *sb, u64 index, int length,
 	}
 
 	if (compressed) {
+		if (!msblk->stream)
+			goto read_failure;
 		length = squashfs_decompress(msblk, bh, b, offset, length,
 			output);
 		if (length < 0)