RFR: addressing a NPE in the compiler due to empty warnStack [v3]

[Vicente Romero]

> Fixing a compiler crash plus other review comments from Maurizio

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  addressing only the crash

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/663/files
  - new: https://git.openjdk.java.net/valhalla/pull/663/files/409de082..b0d5f458

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=663&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=663&range=01-02

  Stats: 136 lines in 3 files changed: 3 ins; 90 del; 43 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/663.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/663/head:pull/663

PR: https://git.openjdk.java.net/valhalla/pull/663