Make sure all_lib_deps_by_context doesn't report invalid contexts

[ghost]

This fixes a bug reported by @samoht on slack

[samoht]

That seems to work for me:

$ jbuilder external-lib-deps --missing --dev @runtest
File "bin/main.ml", line 766, characters 28-34: Assertion failed
Backtrace:
Raised at file "bin/main.ml", line 766, characters 28-40
Called from file "map.ml", line 315, characters 19-42
Called from file "map.ml", line 315, characters 26-41
Called from file "bin/main.ml", line 760, characters 9-1023
Called from file "src/fiber/fiber.ml", line 359, characters 6-13

$ opam pin add jbuilder [email protected]:ocaml/dune.git#pull/723/head -y
$ jbuilder external-lib-deps --missing --dev @runtest
$