sanityCheck: only calculate siblingHash if necessary, refactor and re…

…duce nesting

Only calculate siblingHash to avoid wasting cycles when unneeded.
Also while here, reverse conditional the code to reduce nesting and
to make reading much simpler.

Fixes celestiaorg#56

proofs.go

@@ -42,16 +42,16 @@ func (proof *SparseMerkleProof) sanityCheck(th *treeHasher) bool {
 	}
 
 	// Check that the sibling data hashes to the first side node if not nil
-	if proof.SiblingData != nil {
-		siblingHash := th.digest(proof.SiblingData)
-		if proof.SideNodes != nil && len(proof.SideNodes) > 0 {
-			if !bytes.Equal(proof.SideNodes[0], siblingHash) {
-				return false
-			}
-		}
+	if proof.SiblingData == nil {
+		return true
 	}
 
-	return true
+	if len(proof.SideNodes) == 0 {
+		return true
+	}
+
+	siblingHash := th.digest(proof.SiblingData)
+	return bytes.Equal(proof.SideNodes[0], siblingHash)
 }
 
 // SparseCompactMerkleProof is a compact Merkle proof for an element in a SparseMerkleTree.