store: fix null reference from DerivationGoal::waiteeDone

This happened during a PathSubstitutionGoal of a .drv file:

substitution of '/tmp/jade/nix-test/ca/eval-store/store/1lj7lsq5y0f25mfbnq6d3zd0bw5ay33n-dependencies-input-2.drv'

What happened here is that since PathSubstitutionGoal is not a
DerivationGoal, in production builds, the UB was not caught, since it
would early-exit from failing a dynamic_cast to DerivationGoal * on the
very next line, but before the null reference was ever used.

This was nonetheless UB. The fix should be to just rearrange the two
lines; I don't think there is a further bug there, since *substituting a
.drv* **necessarily** means you cannot have the representation of
the derivation as would be necessary for drv to not be null there.

Test failure:

++(eval-store.sh:12) _RR_TRACE_DIR=/home/jade/.local/share/rr rr record -- nix build -f dependencies.nix --eval-store /tmp/jade/nix-test/ca/eval-store/eval-store -o /tmp/jade/nix-test/ca/eval-store/result
don't know how to build these paths:
  /tmp/jade/nix-test/ca/eval-store/store/6y51mf0p57ggipgab6hdjabbvplzsicq-dependencies-top.drv
copying 1 paths...
copying path '/tmp/jade/nix-test/ca/eval-store/store/8027afyvqb87y1sf5xhdkqsflqn1ziy8-dependencies.builder0.sh' to 'local'...
copying 1 paths...
copying path '/tmp/jade/nix-test/ca/eval-store/store/7r5pqyncvfgrryf9gzy1z56z3xigi61x-builder-dependencies-input-0.sh' to 'local'...
copying 1 paths...
copying path '/tmp/jade/nix-test/ca/eval-store/store/nhmgm87zlqy3ks96dxrn7l37b72azi99-builder-dependencies-input-1.sh' to 'local'...
copying 1 paths...
copying path '/tmp/jade/nix-test/ca/eval-store/store/nq4qa2j6y8ajqazlfq6h46ck637my1n6-builder-dependencies-input-2.sh' to 'local'...
copying 1 paths...
copying path '/tmp/jade/nix-test/ca/eval-store/store/6vh0vna9l5afck01y7iaks3hm9ikwqyj-builder-fod-input.sh' to 'local'...
building '/tmp/jade/nix-test/ca/eval-store/store/gy91pqymf2nc5v7ld1bad94xpwxdi25s-dependencies-input-0.drv'...
building '/tmp/jade/nix-test/ca/eval-store/store/w7wlkjx97ivmnrymkac5av3nyp94hzvq-dependencies-input-1.drv'...
../src/libstore/build/derivation-goal.cc:1556:22: runtime error: reference binding to null pointer of type 'Derivation'
    0 0x734ba59a6886 in nix::DerivationGoal::waiteeDone(std::shared_ptr<nix::Goal>, nix::Goal::ExitCode) /home/jade/lix/lix2/build/src/libstore/build/derivation-goal.cc:1556:12
    1 0x734ba59c0962 in nix::Goal::amDone(nix::Goal::ExitCode, std::optional<nix::Error>) /home/jade/lix/lix2/build/src/libstore/build/goal.cc:95:25
    2 0x734ba5a1c44a in nix::PathSubstitutionGoal::done(nix::Goal::ExitCode, nix::BuildResult::Status, std::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>>) /home/jade/lix/lix2/build/src/libstore/build/substitution-goal.cc:38:5
    3 0x734ba5a1b454 in nix::PathSubstitutionGoal::init() /home/jade/lix/lix2/build/src/libstore/build/substitution-goal.cc:56:9
    4 0x734ba5a2a6c6 in nix::Worker::run(std::set<std::shared_ptr<nix::Goal>, nix::CompareGoalPtrs, std::allocator<std::shared_ptr<nix::Goal>>> const&) /home/jade/lix/lix2/build/src/libstore/build/worker.cc:320:23
    5 0x734ba59b93d8 in nix::Store::buildPathsWithResults(std::vector<nix::DerivedPath, std::allocator<nix::DerivedPath>> const&, nix::BuildMode, std::shared_ptr<nix::Store>) /home/jade/lix/lix2/build/src/libstore/build/entry-points.cc:60:12
    6 0x734ba663c107 in nix::Installable::build2(nix::ref<nix::Store>, nix::ref<nix::Store>, nix::Realise, std::vector<nix::ref<nix::Installable>, std::allocator<nix::ref<nix::Installable>>> const&, nix::BuildMode) /home/jade/lix/lix2/build/src/libcmd/installables.cc:637:36

Change-Id: Id0e651e480bebf6356733b01bc639e9bb59c7bd0

src/libstore/build/derivation-goal.cc

@@ -1553,11 +1553,12 @@ void DerivationGoal::waiteeDone(GoalPtr waitee, ExitCode result)
     Goal::waiteeDone(waitee, result);
 
     if (!useDerivation) return;
-    auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
 
     auto * dg = dynamic_cast<DerivationGoal *>(&*waitee);
     if (!dg) return;
 
+    auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
+
     auto * nodeP = fullDrv.inputDrvs.findSlot(DerivedPath::Opaque { .path = dg->drvPath });
     if (!nodeP) return;
     auto & outputs = nodeP->value;