CombineGVCFs --break-bands-at-multiples-of can generate invalid intermediate intervals

[cmnbroad]

This happens whenever the start position of an interval for which intermediate bands must be created is less than the value of break-bands-at-multiples-of. For example, an input reference block record with a start position (say 1) that is less than the value of of break-bands-at-multiples-of (say 10000) would result in the invalid intermediate band interval:

java.lang.IllegalArgumentException: Invalid interval. Contig:chr21 start:-1 end:-1

	at org.broadinstitute.hellbender.utils.Utils.validateArg(Utils.java:687)
	at org.broadinstitute.hellbender.utils.SimpleInterval.validatePositions(SimpleInterval.java:61)
	at org.broadinstitute.hellbender.utils.SimpleInterval.<init>(SimpleInterval.java:37)
	at org.broadinstitute.hellbender.tools.walkers.CombineGVCFs.createIntermediateVariants(CombineGVCFs.java:191)
	at org.broadinstitute.hellbender.tools.walkers.CombineGVCFs.apply(CombineGVCFs.java:134)

This doesn't happen in GATK3.