Address PR review comments

set `cache->chain` to NULL,
instead of setting the whole structure to zero.
diff --git a/library/ssl_cache.c b/library/ssl_cache.c
index d34bc3d..47867f1 100644
--- a/library/ssl_cache.c
+++ b/library/ssl_cache.c
@@ -43,11 +43,6 @@
 
 #include <string.h>
 
-/* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
-}
-
 void mbedtls_ssl_cache_init( mbedtls_ssl_cache_context *cache )
 {
     memset( cache, 0, sizeof( mbedtls_ssl_cache_context ) );
@@ -326,8 +321,7 @@
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_free( &cache->mutex );
 #endif
-
-    mbedtls_zeroize( cache, sizeof(mbedtls_ssl_cache_context) );
+    cache->chain = NULL;
 }
 
 #endif /* MBEDTLS_SSL_CACHE_C */