zstd: Revert "Disallow 0 sized compressed blocks" (#527)

Fixes #526

Bonus, use `ignorecrc` for disabling crc checks in while fuzzing.

zstd/blockdec.go

@@ -167,9 +167,9 @@ func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
 			}
 			return ErrCompressedSizeTooBig
 		}
-		// Empty compressed blocks not valid in practice,
-		// see https://github.com/facebook/zstd/issues/3090
-		if cSize < 3 {
+		// Empty compressed blocks must at least be 2 bytes
+		// for Literals_Block_Type and one for Sequences_Section_Header.
+		if cSize < 2 {
 			return ErrBlockTooSmall
 		}
 	case blockTypeRaw:

zstd/fuzz.go

@@ -1,5 +1,5 @@
-//go:build gofuzz
-// +build gofuzz
+//go:build ignorecrc
+// +build ignorecrc
 
 // Copyright 2019+ Klaus Post. All rights reserved.
 // License information can be found in the LICENSE file.

zstd/fuzz_none.go

@@ -1,5 +1,5 @@
-//go:build !gofuzz
-// +build !gofuzz
+//go:build !ignorecrc
+// +build !ignorecrc
 
 // Copyright 2019+ Klaus Post. All rights reserved.
 // License information can be found in the LICENSE file.