Add undefined callback check to websocket provider response queue

[wbt]

Defensive programming per last sentence of paragraph 8 in #3573 (comment).

The now-doubled call to .get(id) seems like it might be a little inefficient. I'm not sure how that compares to calling .get(id) once and assigning the result a temporary variable in terms of efficiency, but if someone wants to put some rigor into that selection it'd be helpful.

I don't believe that any corresponding documentation changes are necessary.

A corresponding change in 2.x might be a good idea.

Description

Fixes #3573

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

This is as complete as of initial PR submission as the original submitter will get it:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build-all and tested the resulting files from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.
• The browser visual inspection check looks ok: sudden-playground.surge.sh

[coveralls]

Coverage increased (+0.003%) to 89.858% when pulling a182d7a on wbt:patch-1 into 076fd46 on ethereum:1.x.

[wbt]

For what it's worth, I don't think the failing tests or timeout are related to this commit, nor do I think this commit can be accurately credited with an increase in test coverage.

[cgewecke]

LGTM! Thanks @wbt.

[ryanio]

LGTM, thanks!