missing BOOST_FALLTHROUGH in a few cases

[RoyBellingan]

After seeing a few test failing for #1071 I added a few more check in my local build, and those 3 popped out

[cppalliance-bot]

An automated preview of the documentation is available at https://1073.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.70%. Comparing base (c02d872) to head (e3531d3).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@ Coverage Diff @@ ## develop #1073 +/- ## ======================================== Coverage 93.70% 93.70% ======================================== Files 91 91 Lines 9139 9139 ======================================== Hits 8564 8564 Misses 575 575 

Files with missing lines	Coverage Δ
include/boost/json/basic_parser_impl.hpp	98.50% <ø> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1073/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1073.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1073/pullrequest.html

[grisumbras]

Thank you for your contribution.

[RoyBellingan]

Each Journey begin with one step!