Use plain memset() in context init functions

We call xxx_init() on a structure when it has been freshly allocated (on the
stack or heap).

At this point it contains random-looking data none of which should be
sensitive, as all sensitive data is wiped using mbedtls_platform_zeroize()
when we're done using it and the memory area is going to be reclaimed (by
exiting the function or free()ing the buffer).
diff --git a/library/ccm.c b/library/ccm.c
index a21e3c3..9258082 100644
--- a/library/ccm.c
+++ b/library/ccm.c
@@ -66,7 +66,7 @@
 void mbedtls_ccm_init( mbedtls_ccm_context *ctx )
 {
     CCM_VALIDATE( ctx != NULL );
-    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_ccm_context ) );
+    memset( ctx, 0, sizeof( mbedtls_ccm_context ) );
 }
 
 int mbedtls_ccm_setkey( mbedtls_ccm_context *ctx,