fix(core): set default value for do_not_restart

otherwise if _handle_single_message raises an exception (which is
fortunately not something that should generally happen), the
finally-block would fail on "referenced before assignment"

core/src/trezor/wire/__init__.py

@@ -216,6 +216,7 @@ async def handle_session(iface: WireInterface, session_id: int) -> None:
                 msg = next_msg
                 next_msg = None
 
+            do_not_restart = False
             try:
                 do_not_restart = await _handle_single_message(ctx, msg)
             except context.UnexpectedMessage as unexpected: