OSX: don't try to call *nrm2 blas calls if incx < 1 or N < 1
OSX: don't try to call *nrm2 blas calls if incx < 1 or N < 1

Because calling nrm2 with negative incx will lead to a crash on
OSX. See issue #37.

kknox/clBLAS,linan7788626/clBLAS,devfreesg/clBLAS,clMathLibraries/clBLAS,tingxingdong/clean,guacamoleo/clBLAS,TimmyLiu/clBLAS,hemmingway/clBLAS,BenjaminCoquelle/clBLAS,guacamoleo/clBLAS,linan7788626/clBLAS,guacamoleo/clBLAS,tingxingdong/clean,tingxingdong/clean,TimmyLiu/clBLAS,ghisvail/clBLAS,guacamoleo/clBLAS,hughperkins/clBLAS,hemmingway/clBLAS,arrayfire/clBLAS,devfreesg/clBLAS,tmagomedov/clBLAS,lunochod/clBLAS,linan7788626/clBLAS,tmagomedov/clBLAS,lunochod/clBLAS,devfreesg/clBLAS,hemmingway/clBLAS,devfreesg/clBLAS,clMathLibraries/clBLAS,tmagomedov/clBLAS,kknox/clBLAS,ghisvail/clBLAS,hughperkins/clBLAS,arrayfire/clBLAS,hemmingway/clBLAS,linan7788626/clBLAS,TimmyLiu/clBLAS,lunochod/clBLAS,tmagomedov/clBLAS,clMathLibraries/clBLAS,arrayfire/clBLAS,arrayfire/clBLAS,clMathLibraries/clBLAS,BenjaminCoquelle/clBLAS,kknox/clBLAS,hughperkins/clBLAS,BenjaminCoquelle/clBLAS,ghisvail/clBLAS,lunochod/clBLAS,TimmyLiu/clBLAS,BenjaminCoquelle/clBLAS,tingxingdong/clean,kknox/clBLAS,hughperkins/clBLAS,ghisvail/clBLAS