Reject passphrase config option without argument

[Errordog2]

Summary

Fixes #696 by making the passphrase configuration-file option follow the same missing-argument path as other argument-requiring options.

When passphrase appears without an argument, dooption() now jumps to the existing
eedarg handler before calling passphrase_entry_parse(), avoiding the NULL dereference reported in the issue.

Validation

• Inspected the current master implementation and confirmed passphrase was the only path here calling passphrase_entry_parse(conf_arg, ...) without a prior conf_arg == NULL guard.
• Confirmed the PR diff is limited to ar/bsdtar.c and adds only the existing-style conf_arg == NULL / goto needarg check.

I was unable to run a local build in this environment because repeated git clone attempts to GitHub failed with network connection resets/timeouts, so I submitted the minimal patch through the GitHub API.

[Errordog2]

Additional validation from this environment:

• Downloaded the PR branch via GitHub codeload after git clone kept failing with connection resets.
• Re-inspected ar/bsdtar.c in the downloaded source archive.
• Confirmed the passphrase branch now checks conf_arg == NULL and jumps to the existing
  eedarg handler before passphrase_entry_parse().
• Confirmed the branch contains the same existing missing-argument pattern used by nearby argument-requiring options.

I still cannot run the full autotools build/test suite locally here because the available WSL environment lacks the needed build tools, but the patch remains a minimal control-flow guard.

[gperciva]

Closing and re-opening to trigger CI run.