[base] 'close' callback may not use `stream->memory`.

The documentation for `FT_StreamRec::memory` states that it 'shouldn't be
touched by stream implementations'.  This is true even for internal
implementations of the 'close' callback, since it is not guaranteed that
`memory` will even be set when the 'close' callback occurs.

* src/base/ftobjs.c (new_memory_stream): stash current `memory` in
`stream->descriptor`.
(memory_stream_close): Use it.

src/base/ftobjs.c

@@ -1672,7 +1672,7 @@
   static void
   memory_stream_close( FT_Stream  stream )
   {
-    FT_Memory  memory = stream->memory;
+    FT_Memory  memory = (FT_Memory)stream->descriptor.pointer;
 
 
     FT_FREE( stream->base );
@@ -1709,7 +1709,8 @@
 
     FT_Stream_OpenMemory( stream, base, size );
 
-    stream->close = close;
+    stream->descriptor.pointer = memory;
+    stream->close              = close;
 
     *astream = stream;