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 1cb71bf..0cad148 100644
--- a/library/ssl_cache.c
+++ b/library/ssl_cache.c
@@ -44,12 +44,6 @@
 #define polarssl_free       free
 #endif
 
-
-/* Implementation that should never be optimized out by the compiler */
-static void polarssl_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
-}
-
 void ssl_cache_init( ssl_cache_context *cache )
 {
     memset( cache, 0, sizeof( ssl_cache_context ) );
@@ -331,7 +325,7 @@
     polarssl_mutex_free( &cache->mutex );
 #endif
 
-    polarssl_zeroize( cache, sizeof(ssl_cache_context) );
+    cache->chain = NULL;
 }
 
 #endif /* POLARSSL_SSL_CACHE_C */