Fix integer overflow in mbedtls_base64_decode()

Fix potential integer overflows in the function mbedtls_base64_decode().
This overflow would mainly be exploitable in 32-bit systems and could
cause buffer bound checks to be bypassed.
diff --git a/ChangeLog b/ChangeLog
index 5856f41..e8fa013 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,6 +12,8 @@
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
    * Fixed potential arithmetic overflow in mbedtls_md2_update() that could
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflow in mbedtls_base64_decode() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
 
 = mbed TLS 2.1.6 branch released 2016-10-17