Slang-netlist: add guard around port expression

[jameshanlon]

Fixes #919.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.00%. Comparing base (46d9e68) to head (7c7b41d).

Additional details and impacted files

[](https://app.codecov.io/gh/MikePopoloski/slang/pull/921?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski) ```diff @@ Coverage Diff @@ ## master #921 +/- ## ======================================= Coverage 94.00% 94.00% ======================================= Files 189 189 Lines 48390 48390 ======================================= Hits 45490 45490 Misses 2900 2900 ``` ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/MikePopoloski/slang/pull/921?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/MikePopoloski/slang/pull/921?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski). Last update [46d9e68...7c7b41d](https://app.codecov.io/gh/MikePopoloski/slang/pull/921?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Popoloski).