Fix bug in ssl_write_supported_elliptic_curves_ext

Passing invalid curves to mbedtls_ssl_conf_curves potentially could caused a
crash later in ssl_write_supported_elliptic_curves_ext. #373
diff --git a/ChangeLog b/ChangeLog
index e9b6790..11cc522 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,8 @@
    * Fix issue in Makefile that prevented building using armar. #386
    * Fix memory leak that occured only when ECJPAKE was enabled and ECDHE and
      ECDSA was disabled in config.h . The leak didn't occur by default.
+   * Fix issue that caused a crash if invalid curves were passed to
+     mbedtls_ssl_conf_curves. #373
 
 Changes
    * On ARM platforms, when compiling with -O0 with GCC, Clang or armcc5,