Updated BED entry sanity checking.
Updated BED entry sanity checking.

	Forced an exit if negative coordinates are encountered.

jmarshall/bedtools2,jmarshall/bedtools2,jmarshall/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,jmarshall/bedtools2,jmarshall/bedtools2