mbedtls: Update to upstream version 2.12.0

_WIN32_WINNT redefinition fix is no longer needed as it was merged
upstream. PR 1453 is still not merged, diff updated to current state.

thirdparty/mbedtls/library/memory_buffer_alloc.c

@@ -518,7 +518,9 @@ void mbedtls_memory_buffer_alloc_status( void )
                       heap.alloc_count, heap.free_count );
 
     if( heap.first->next == NULL )
+    {
         mbedtls_fprintf( stderr, "All memory de-allocated in stack buffer\n" );
+    }
     else
     {
         mbedtls_fprintf( stderr, "Memory currently allocated:\n" );