[C++] Verify missing fields when walking Flatbuffers data

[asfimport]

This will fix some of the issues detected by OSS-Fuzz.

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] NULL pointer dereference bug (incorporates)
• [C++] NULL pointer dereference bug (supercedes)

PRs and other links:

• GitHub Pull Request #13
• GitHub Pull Request #6293

_{Note: This issue was originally created as ARROW-7691. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 6293
#6293