fix segfault when a read segment only has 1 event
fix segfault when a read segment only has 1 event

mateidavid/nanopolish,mateidavid/nanopolish,mateidavid/nanopolish,jts/nanopolish,mateidavid/nanopolish,jts/nanopolish,mateidavid/nanopolish,jts/nanopolish,jts/nanopolish,jts/nanopolish