fix memory leak

Change-Id: I6683ab0d39dbbb5c0ba0958d72f0a1bbfab47164
Reviewed-on: https://gerrit.libreoffice.org/43954
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

comphelper/source/misc/docpasswordhelper.cxx

@@ -336,6 +336,8 @@ Sequence< sal_Int8 > DocPasswordHelper::GetXLHashAsSequence(
 
         // Erase KeyData array and leave.
         rtl_secureZeroMemory (pKeyData, sizeof(pKeyData));
+
+        rtl_digest_destroy(hDigest);
     }
 
     return aResultKey;