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/cipher.c b/library/cipher.c
index 47dfe24..53ed9b3 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -156,7 +156,7 @@
 void mbedtls_cipher_init( mbedtls_cipher_context_t *ctx )
 {
     CIPHER_VALIDATE( ctx != NULL );
-    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_cipher_context_t ) );
+    memset( ctx, 0, sizeof( mbedtls_cipher_context_t ) );
 }
 
 void mbedtls_cipher_free( mbedtls_cipher_context_t *ctx )