Update the _matchAfterIndex variable after setting new filter mappings

[kshep92]

Updates the _matchAfterIndex to the correct value based on how many filter mappings the user added to the ServletHandler. Failing to do so produces an IndexOutOfBoundsException when adding subsequent filter mappings.

[joakime]

Thank you for the PR, but ...

First, we need an ECA to merge this. See CONTRIBUTING.md
Second, we need a testcase that shows the issue (as this often shows a different place, or more places, that the issue should be corrected in)

[kshep92]

Thank you for your guidance @joakime I was unable to create an account on the Eclipse foundation website as it's showing me an error page after submitting the registration form. I will try again later. In the meantime, I've attached a test case for examination.
jetty-test-case.zip

[janbartel]

@kshep92 thanks for the PR, you are on the right track, but I think we need to do a full reset of both indexes. I've opened PR #12734 .