go/tools/go/gcimporter: avoid possible endless loops in case of errors
go/tools/go/gcimporter: avoid possible endless loops in case of errors

R=adonovan
CC=golang-dev
https://golang.org/cl/39630045

sshetty10/tools,springning/tools,maddyonline/tools,syreclabs/go-tools,jdhenke/tools,runcom/tools,runcom/tools,golang/tools,magiccao/tools,nicot/tools,dmitris/tools,runcom/tools,magiccao/tools,cw2018/tools,glasser/golang-tools,dmitris/tools,maddyonline/tools,4ad/tools,josharian/tools,matryer/tools,dmitris/tools,cmars/tools,syreclabs/go-tools,matryer/tools,golang/tools,matryer/tools,argandas/tools,cmars/tools,golang/tools,springning/tools,springning/tools,argandas/tools,dmitris/tools,golang/tools,sshetty10/tools,maddyonline/tools,josharian/tools,magiccao/tools,4ad/tools,glasser/golang-tools,argandas/tools,jdhenke/tools,jdhenke/tools,glasser/golang-tools,nicot/tools,maddyonline/tools,cw2018/tools,cmars/tools,springning/tools,cmars/tools,syreclabs/go-tools,syreclabs/go-tools,argandas/tools,golang/tools,sshetty10/tools,magiccao/tools,cw2018/tools,glasser/golang-tools,syreclabs/go-tools,matryer/tools,jdhenke/tools,runcom/tools,cw2018/tools,sshetty10/tools,4ad/tools,nicot/tools,josharian/tools,syreclabs/go-tools,josharian/tools,nicot/tools