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/entropy.c b/library/entropy.c
index 4e29b31..8f28733 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -65,7 +65,7 @@
 void mbedtls_entropy_init( mbedtls_entropy_context *ctx )
 {
     ctx->source_count = 0;
-    mbedtls_platform_memset( ctx->source, 0, sizeof( ctx->source ) );
+    memset( ctx->source, 0, sizeof( ctx->source ) );
 
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &ctx->mutex );