Fix socket shutdown bug

[cgay]

  Test test-repeated-start-stop-with-connection: PASSED in 5.058714s and 1012KiB
Cannot add an element with key {<byte-char-tcp-socket>} to a sequence of type {<class>: <vector>}
Backtrace:
  error:dylan:dylan##0 + 0x2a
  convert-accumulator-as:internal:dylan##2 + 0x333
  map-as-one:internal:dylan##6 + 0x1cb
  shutdown-all-sockets:sockets-internals:network##0 + 0x198
  stop-sockets:sockets-internals:network + 0x29
  ...
  exit-application:common-extensions:common-dylan + 0xa
  run-test-application:testworks:testworks + 0x458

See #1743 (comment)

I don't think this needs a release note because it just fixes a problem that was created after the most recent release.