list_reader_selector: Prevent infinite loop
list_reader_selector: Prevent infinite loop

In case the readers are empty.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20170811153142.8926-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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