tests/double-response: harden against missing error arg3
tests/double-response: harden against missing error arg3

chenwenbin928/tchannel,chenwenbin928/tchannel,benfleis/tchannel,hustxiaoc/tchannel,sasa233/tchannel,chenwenbin928/tchannel,vanloswang/tchannel,RyanTech/tchannel,savaki/tchannel,RyanTech/tchannel,hustxiaoc/tchannel,benfleis/tchannel,bunnyblue/tchannel,sasa233/tchannel,hustxiaoc/tchannel,sasa233/tchannel,Zirpon/tchannel,Zirpon/tchannel,bunnyblue/tchannel,RyanTech/tchannel,savaki/tchannel,vanloswang/tchannel,i/tchannel,Zirpon/tchannel,i/tchannel,vanloswang/tchannel,savaki/tchannel,bunnyblue/tchannel,i/tchannel,benfleis/tchannel