RFR: 8288567: [macOS] NPE at sun.lwawt.macosx.CPlatformComponent.setBounds

[Maxim Kartashev]

Even though I have not been able to reproduce the crash by hand, the problem and the fix are fairly obvious: 
`CWarningWindow` doesn't have a peer, so `platformWindow.getPeer()` can be null in that case. The fix is to accomodate for that in `CPlatformComponent.setBounds()`. Similar provisions exist elsewhere in the vicinity (for ex. `CPlatformLWWindow.setBounds()`, `CPlatformWindow.maximize()`, etc).

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Commit messages:
 - 8288567: [macOS] NPE at sun.lwawt.macosx.CPlatformComponent.setBounds

Changes: https://git.openjdk.org/jdk/pull/9184/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9184&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288567
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9184.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9184/head:pull/9184

PR: https://git.openjdk.org/jdk/pull/9184