Fixed Array Bounds for Fragment

Fixed array bound from FRAGMENT_LENGTH (which is 2187 in trytes) to HASH_LENGTH (which is 243 in trits).

Author: phil-zxx

iota/crypto/types.py

@@ -189,8 +189,8 @@ def get_digest(self):
       sponge.absorb(key_fragment)
       sponge.squeeze(hash_trits)
 
-      fragment_start  = i * FRAGMENT_LENGTH
-      fragment_end    = fragment_start + FRAGMENT_LENGTH
+      fragment_start  = i * HASH_LENGTH
+      fragment_end    = fragment_start + HASH_LENGTH
 
       digest[fragment_start:fragment_end] = hash_trits