[linter] Avoid unreachable code in shell scripts

Summary: Reported by Shellcheck SC2317. Note that some of the reports are false positive and has been explicitely ignored.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13842