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://codereview.appspot.com/39630045

chenjianlong/go.tools,chenjianlong/go.tools,lattera/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,lattera/go.tools