recover_segment(): don't assume we have an fd for segment

Suggested by @ThomasWaldmann.  Avoiding a complex assumption
should make the code easier to understand and maintain.

(Technically we do have an fd for the segment, because
the only caller opens the segment and checks it before
calling for repair.)

attic/repository.py

@@ -515,7 +515,9 @@ class LoggedIO(object):
             header = fd.read(self.header_fmt.size)
 
     def recover_segment(self, segment, filename):
-        self.fds.pop(segment).close()
+        fd = self.fds.pop(segment)
+        if fd is not None:
+            fd.close()
         # FIXME: save a copy of the original file
         with open(filename, 'rb') as fd:
             data = memoryview(fd.read())