fix: fix goal index search in setgoal function

[tkimura4]

Related Issue(required)

Description(required)

Fix searching of goal index in behavior_path_planner.

Review Procedure(required)

Related PR(optional)

tier4/AutowareArchitectureProposal.iv#786

Pre-Review Checklist for the PR Author

PR Author should check the checkboxes below when creating the PR.

• Read commit-guidelines
• Assign PR to reviewer

If you are adding new package following items are required:

• Documentation with description of the package is available
• A sample launch file and parameter file are available if the package contains executable nodes

Checklist for the PR Reviewer

Reviewers should check the checkboxes below before approval.

• Commits are properly organized and messages are according to the guideline
• PR title describes the changes

Post-Review Checklist for the PR Author

PR Author should check the checkboxes below before merging.

• All open points are addressed and tracked via issues or tickets

CI Checks

• Build and test for PR / build-and-test-pr: Required to pass before the merge.
• Build and test for PR / clang-tidy-pr: NOT required to pass before the merge. It is up to the reviewer(s). Found false positives? See the [guidelines][clang-tidy-guidelines].
• Check spelling: NOT required to pass before the merge. It is up to the reviewer(s). See here if you want to add some words to the spell check dictionary.

[wep21]

LGTM