[libmicrohttpd] Correct handling of the MHD_ContentReaderCallback

[Martin Velek]

Hello,

I am trying to write a robust MHD_ContentReaderCallback callback.
However I cannot imagine how to handle a situation when MHD calls my
callback with the provided buffer size equal to 0 (I am running
internal select mode). The documentation does not state it as an
impossible situation.

I can't return 0 because of the internal select mode.
I can't return MHD_CONTENT_READER_END_* because I would like to handle
the request.


Best
Martin