Close a race in the round robin director - for real now
Close a race in the round robin director - for real now

The previous code allowed the compiler to re-read nxt from
rr->nxt which could have been incremented cocurrently.

Fixes #2378

feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache