Fix biweight_midcovariance when mad=0
Fix biweight_midcovariance when mad=0

lpsinger/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,mhvk/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,pllim/astropy,lpsinger/astropy,StuartLittlefair/astropy,saimn/astropy,dhomeier/astropy,astropy/astropy,lpsinger/astropy,saimn/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,saimn/astropy,dhomeier/astropy,pllim/astropy,StuartLittlefair/astropy,saimn/astropy,saimn/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,astropy/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,pllim/astropy