#79 Add null checks and small improvements

Author: martti007

src/main/java/org/unicitylabs/sdk/api/InclusionCertificate.java

@@ -28,6 +28,10 @@ private InclusionCertificate(byte[] bitmap, List<DataHash> siblings) {
 
   public static InclusionCertificate create(SparseMerkleTreeRootNode root, byte[] key) {
     Objects.requireNonNull(root, "root cannot be null");
+    Objects.requireNonNull(key, "key cannot be null");
+    if (key.length != 32) {
+      throw new IllegalArgumentException("Key must be 32 bytes long.");
+    }
 
     ArrayList<DataHash> siblings = new ArrayList<>();
     byte[] bitmap = new byte[InclusionCertificate.BITMAP_SIZE];

src/main/java/org/unicitylabs/sdk/smt/SparseMerkleTreePathUtils.java

@@ -90,6 +90,6 @@ public static int getBitAtDepth(byte[] data, int depth) {
     }
     int byteIndex = depth / 8;
     int bitInByte = depth % 8;
-    return (data[byteIndex] >> (7 - bitInByte)) & 1;
+    return ((data[byteIndex] & 0xff) >> (7 - bitInByte)) & 1;
   }
 }