lib/protocol: Use constants instead of init time hashing (fixes #5624) (#5625)

This constructs the map of hashes of zero blocks from constants instead
of calculating it at startup time. A new test verifies that the map is
correct.

lib/protocol/protocol_test.go

@@ -4,6 +4,7 @@ package protocol
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
@@ -610,3 +611,13 @@ func TestIsEquivalent(t *testing.T) {
 		}
 	}
 }
+
+func TestSha256OfEmptyBlock(t *testing.T) {
+	// every block size should have a correct entry in sha256OfEmptyBlock
+	for blockSize := MinBlockSize; blockSize <= MaxBlockSize; blockSize *= 2 {
+		expected := sha256.Sum256(make([]byte, blockSize))
+		if sha256OfEmptyBlock[blockSize] != expected {
+			t.Error("missing or wrong hash for block of size", blockSize)
+		}
+	}
+}