Proxy: Fix unready forwarded requests causing crash

See merge request !490
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

02:04:45

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:30

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:28