Replace malloc with calloc

- platform layer currently broken (not adapted yet)
- memmory_buffer_alloc too
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 466e337..bd28406 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1986,7 +1986,7 @@
  *
  * This module enables abstraction of common (libc) functions.
  */
-#define MBEDTLS_PLATFORM_C
+//#define MBEDTLS_PLATFORM_C
 
 /**
  * \def MBEDTLS_RIPEMD160_C