Fixed minor bug in computing b_next in gemm.
Fixed minor bug in computing b_next in gemm.

arm-hpc/blis,audiofilter/blis,rwl/blis,rwl/blis,rwl/blis,scibuilder/blis,xianyi/blis,rwl/blis,xianyi/blis,scibuilder/blis,arm-hpc/blis,scibuilder/blis,arm-hpc/blis,xianyi/blis,audiofilter/blis,audiofilter/blis,audiofilter/blis,xianyi/blis,scibuilder/blis,arm-hpc/blis