Fix integer overflow 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.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index fc240c3..76cd298 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -27,6 +27,8 @@
      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 2.4.1 branch released 2016-12-13