patch 9.1.0112: Remove undo information, when cleaning quickfix buffer

Problem:  When the quickfix buffer has been modified an autocommand
          may invalidate the undo stack (kawarimidoll)
Solution: When clearing the quickfix buffer, also wipe the undo stack

fixes: #13905
closes: #13928

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/quickfix.c

@@ -4829,8 +4829,18 @@ qf_fill_buffer(qf_list_T *qfl, buf_T *buf, qfline_T *old_last, int qf_winid)
 	}
 
 	// delete all existing lines
+	//
+	// Note: we cannot store undo information, because
+	// qf buffer is usually not allowed to be modified.
+	//
+	// So we need to clean up undo information
+	// otherwise autocommands may invalidate the undo stack
 	while ((curbuf->b_ml.ml_flags & ML_EMPTY) == 0)
 	    (void)ml_delete((linenr_T)1);
+
+	// Remove all undo information
+	u_blockfree(curbuf);
+	u_clearall(curbuf);
     }
 
     // Check if there is anything to display