Fix auto argument passing for more auto arguments than formals

The change in 626200713b didn't account
for when the number of auto arguments is bigger than the number of
formal arguments. This causes the following:

  $ nix-instantiate --eval -E '{ ... }@args: args.foo' --argstr foo foo
  nix-instantiate: src/libexpr/attr-set.hh:55: void nix::Bindings::push_back(const nix::Attr&): Assertion `size_ < capacity_' failed.
  Aborted (core dumped)

src/libexpr/eval.cc

@@ -1348,7 +1348,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
     }
 
     Value * actualArgs = allocValue();
-    mkAttrs(*actualArgs, fun.lambda.fun->formals->formals.size());
+    mkAttrs(*actualArgs, std::max(static_cast<uint32_t>(fun.lambda.fun->formals->formals.size()), args.size()));
 
     if (fun.lambda.fun->formals->ellipsis) {
         // If the formals have an ellipsis (eg the function accepts extra args) pass