Optimize unnecessary zeorizing in mbedtls_mpi_copy

Based on a contribution by Alexey Skalozub
(https://github.com/ARMmbed/mbedtls/pull/405).
diff --git a/ChangeLog b/ChangeLog
index 8db0215..25f52c8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.x.x branch released xxxx-xx-xx
+
+Changes
+   * Optimize unnecessary zeroing in mbedtls_mpi_copy. Based on a contribution
+     by Alexey Skalozub.
+
 = mbed TLS 2.7.0 branch released 2018-02-03
 
 Security