Fix args potentially being garbage collected before use in luv_spawn
Fix args potentially being garbage collected before use in luv_spawn

This is one possible fix for the process bug in https://github.com/luvit/luv/issues/510. One other way to fix it would be to just leave the args strings on the stack instead of popping them off. Not sure which is better.

zhaozg/luv,luvit/luv,zhaozg/luv,luvit/luv