Backport 2.1:Fix crash when calling `mbedtls_ssl_cache_free` twice

Set `cache` to zero at the end of `mbedtls_ssl_cache_free` #1104
diff --git a/ChangeLog b/ChangeLog
index da29d70..1df65ed 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,6 +12,8 @@
    * Fix leap year calculation in x509_date_is_valid() to ensure that invalid
      dates on leap years with 100 and 400 intervals are handled correctly. Found
      by Nicholas Wilson. #694
+   * Fix crash when calling mbedtls_ssl_cache_free() twice. Found by
+     MilenkoMitrovic, #1104
 
 = mbed TLS 2.1.9 branch released 2017-08-10