Add missing autoreset in Packer.pack_ext_type

pack_ext_type writes to the internal buffer but never checks
self._autoreset, unlike every other public pack method. This means
it always returns None and the packed data leaks into the output of
the next pack() call, corrupting the serialized stream.

Add the same autoreset pattern used by pack(), pack_map_pairs(),
pack_array_header(), and pack_map_header().

msgpack/fallback.py

@@ -861,6 +861,10 @@ class Packer:
             self._buffer.write(b"\xc9" + struct.pack(">I", L))
         self._buffer.write(struct.pack("B", typecode))
         self._buffer.write(data)
+        if self._autoreset:
+            ret = self._buffer.getvalue()
+            self._buffer = BytesIO()
+            return ret
 
     def _pack_array_header(self, n):
         if n <= 0x0F: