make the flush method re-entrant

Author: mguijarr

prompt_toolkit/output/vt100.py

@@ -669,6 +669,7 @@ def flush(self) -> None:
             return
 
         data = "".join(self._buffer)
+        self._buffer = []
 
         try:
             # Ensure that `self.stdout` is made blocking when writing into it.
@@ -709,8 +710,6 @@ def flush(self) -> None:
             else:
                 raise
 
-        self._buffer = []
-
     def ask_for_cpr(self) -> None:
         """
         Asks for a cursor position report (CPR).