Fix live share jackout error

[bpringe]

What has Changed?

• Check for shared port existence before calling dispose on it

Fixes #856

My Calva PR Checklist

I have:

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the default PR base branch, so that it is not master. (Sorry for the nagging.)
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• Tested the VSIX built from the PR (so, after you've submitted the PR). You'll find the artifacts by clicking Show all checks in the CI section of the PR page, and then Details on the ci/circleci: build test. NB: There is a CircleCI bug that makes the Artifacts hard to find. Please see this issue for workarounds.
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• Referenced the issue I am fixing/addressing in a commit message for the pull request.
  • If I am fixing the issue, I have used GitHub's fixes/closes syntax
  • If I am fixing just part of the issue, I have just referenced it w/o any of the "fixes” keywords.
• Created the issue I am fixing/addressing, if it was not present.

The Calva Team PR Checklist:

Before merging we (at least one of us) have:

• Made sure the PR is directed at the dev branch (unless reasons).
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and tested it there if so.
• Read the source changes.
• Given feedback and guidance on source changes, if needed. (Please consider noting extra nice stuff as well.)
• Tested the VSIX built from the PR (well, if this is a PR that changes the source code.)
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• If need be, had a chat within the team about particular changes.

Ping @PEZ, @kstehn, @cfehse, @bpringe