Replace malloc with calloc

- platform layer currently broken (not adapted yet)
- memmory_buffer_alloc too
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 7663e36..8b993ac 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -38,7 +38,7 @@
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
-#define mbedtls_malloc     malloc
+#define mbedtls_calloc    calloc
 #define mbedtls_free       free
 #endif