RFR: 7903166: jtreg crashes trying to compile .jasm with enablePreview=true

[Evgeny Nikitin]

The bug: [CODETOOLS-7903166](https://bugs.openjdk.java.net/browse/CODETOOLS-7903166)

A small fix targeting crashes that occurs when jtreg compiles non-javac files with enablePreview. Prior the fix, the code crashed trying to insert arguments into javacArgs list using negative index.

Testing: test/preview folder self-tests.

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Commit messages:
 - 7903166: jtreg crashes trying to compile .jasm with enablePreview=true

Changes: https://git.openjdk.java.net/jtreg/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=77&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903166
  Stats: 73 lines in 4 files changed: 71 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/77/head:pull/77

PR: https://git.openjdk.java.net/jtreg/pull/77