RFR: StackChunk only special in null loader.

[Coleen Phillimore]

This adds a check and a test so that only the real StackChunk is treated as StackChunk.

-------------

Commit messages:
 - StackChunk only special in null loader.

Changes: https://git.openjdk.java.net/loom/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=163&range=00
  Stats: 125 lines in 3 files changed: 124 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/loom/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/163/head:pull/163

PR: https://git.openjdk.java.net/loom/pull/163