avoid division by zero in block param computation
avoid division by zero in block param computation

efeguney/gemmlowp,maciekcc/gemmlowp,google/gemmlowp,efeguney/gemmlowp,maciekcc/gemmlowp,petewarden/gemmlowp,efeguney/gemmlowp,google/gemmlowp,google/gemmlowp,petewarden/gemmlowp,maciekcc/gemmlowp,google/gemmlowp,petewarden/gemmlowp,petewarden/gemmlowp