BAMSplitGuesser: fix infloop on some inputs
BAMSplitGuesser: fix infloop on some inputs

Before this, we could return up-1, thus countering the effect of the
++up in the main for loop and looping forever.

tkrishp/Hadoop-BAM,HadoopGenomics/Hadoop-BAM,fnothaft/Hadoop-BAM,cmnbroad/Hadoop-BAM,HadoopGenomics/Hadoop-BAM,ryan-williams/Hadoop-BAM,HadoopGenomics/Hadoop-BAM,fnothaft/Hadoop-BAM,ryan-williams/Hadoop-BAM,cmnbroad/Hadoop-BAM,fnothaft/Hadoop-BAM,tomwhite/Hadoop-BAM