RFR: 7903808: IOOBE in compile action when processing class files only

[Christian Stein]

Please review this fix preventing an `IndexOutOfBoundsException` in the compile action when processing class files only.

As an library used by a test might need `--enable-preview` it is possible that the `insertPos` variable is still assigned with the value `-1`: the `for (String currArg : args) { ... }` loop may exit normally without setting a valid insertion position.

This change checks for `-1` values and use `0` instead in order to prepend `--enable-preview` and `--source N` before any variadic arguments.

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Commit messages:
 - Prevent IOOBE in compile action when processing class files only

Changes: https://git.openjdk.org/jtreg/pull/224/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=224&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903808
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/224.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/224/head:pull/224

PR: https://git.openjdk.org/jtreg/pull/224