Don't assert when discarding unfinished jobs associated with dead servers.
Don't assert when discarding unfinished jobs associated with dead servers.

They may be there because they originally failed to allocate, so they're
not guaranteed to be there in the first place.

Fixes #415

luser/sccache2,glandium/sccache,mozilla/sccache,luser/sccache2,glandium/sccache,glandium/sccache,luser/sccache2,luser/sccache2,mozilla/sccache,mozilla/sccache,glandium/sccache