Don't close self.file before we know we can reopen it

If file permissions change, reopening will fail, and we'll end up
getting an exception, a closed self.file, and a background thread that
will die next time it tries to write to the now-closed file, leaving the
child process writing to a pipe until the pipe buffer fills up at some
point in the future (see also bug #1).

src/zdaemon/zdrun.py

@@ -635,9 +635,10 @@ def copy(self):
                     self.write(os.read(fd, 8192))
 
     def reopen(self):
+        new_file = open(self.filename, 'ab', 0)
         with self.lock:
             self.file.close()
-            self.file = open(self.filename, 'ab', 0)
+            self.file = new_file
             self.write = self.file.write