fix an integer overflow for the Bloom filter

This patch corrects a bug. See the links below.

Link: https://github.com/sebhtml/ray/issues/196
Link: http://permalink.gmane.org/gmane.science.biology.ray-genome-assembler/619
Reported-by Nathaniel Jue <n.jue@uconn.edu>
Link: http://permalink.gmane.org/gmane.science.biology.ray-genome-assembler/627
Reported-by: Ranjeev <ranjeev@um.edu.my>
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>