cmd/cover: check error
cmd/cover: check error

The variable was assigned but not used, as caught by a tool
written by gorden.klaus@gmail.com.

Change-Id: I84ca3d00896287a35561bd122a0cf64212854a86
Reviewed-on: https://go-review.googlesource.com/10610
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

runcom/tools,jdhenke/tools,syreclabs/go-tools,cmars/tools,springning/tools,syreclabs/go-tools,syreclabs/go-tools,springning/tools,runcom/tools,cmars/tools,jdhenke/tools,golang/tools,josharian/tools,josharian/tools,sshetty10/tools,matryer/tools,nicot/tools,sshetty10/tools,magiccao/tools,golang/tools,cmars/tools,sshetty10/tools,josharian/tools,glasser/golang-tools,argandas/tools,matryer/tools,glasser/golang-tools,magiccao/tools,sshetty10/tools,magiccao/tools,jdhenke/tools,nicot/tools,runcom/tools,nicot/tools,springning/tools,golang/tools,glasser/golang-tools,josharian/tools,syreclabs/go-tools,glasser/golang-tools,cw2018/tools,syreclabs/go-tools,cmars/tools,argandas/tools,jdhenke/tools,cw2018/tools,syreclabs/go-tools,argandas/tools,argandas/tools,nicot/tools,matryer/tools,matryer/tools,golang/tools,cw2018/tools,cw2018/tools,magiccao/tools,springning/tools,golang/tools,runcom/tools