Update change log

ChangeLog

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 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Security
+   * Fix an issue in the X.509 module which could lead to a buffer overread
+     during certificate extensions parsing. In case of receiving malformed
+     input (extensions length field equal to 0), an illegal read of one byte
+     beyond the input buffer is made. Found and analyzed by Nathan Crandall.
+
 Bugfix
    * Fix compilation error on C++, because of a variable named new.
      Found and fixed by Hirotaka Niisato in #1783.