Fix auth token conversion dropping a byte

Test: TestDPC
Bug: 34656122
Change-Id: I000b2115e7c7764de18e1f3b3e3d5c16ca4b9443
diff --git a/keystore/include/keystore/keystore_hidl_support.h b/keystore/include/keystore/keystore_hidl_support.h
index e858021..3c64d2a 100644
--- a/keystore/include/keystore/keystore_hidl_support.h
+++ b/keystore/include/keystore/keystore_hidl_support.h
@@ -111,7 +111,7 @@
     result.resize(sizeof(hw_auth_token_t));
     auto pos = result.begin();
     *pos++ = 0;  // Version byte
-    pos = copy_bytes_to_iterator(token.challenge, result.begin());
+    pos = copy_bytes_to_iterator(token.challenge, pos);
     pos = copy_bytes_to_iterator(token.userId, pos);
     pos = copy_bytes_to_iterator(token.authenticatorId, pos);
     pos = copy_bytes_to_iterator(token.authenticatorType, pos);