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 ea55df8..bee652c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -19,6 +19,8 @@
    * Fix issue that caused a hang when generating RSA keys of odd bitlength
    * Fix bug in mbedtls_rsa_rsaes_pkcs1_v15_encrypt that made null pointer
      dereference possible.
+   * 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,