RFR: 8300038: Make new version of JNU_GetStringPlatformChars which checks for null characters

[Michael McMahon]

This PR creates a new version of the JNI utility function JNU_GetStringPlatformChars called JNU_GetStringPlatformCharsStrict, which performs additional validation of the returned string, namely that it does not contain any embedded NULL characters. If any such characters are found the function returns NULL with an IAE pending. The change also switches usage in the networking native code to use the new function.

This cautious approach was taken rather than changing the behavior of the existing function as each native code area needs to review the effect of making the switch. Otherwise, surprising behavior changes might occur (eg undocumented IAE being thrown to user code instead of some other exception).

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Commit messages:
 - test update
 - Merge branch 'master' into nullStrings
 - exception message update
 - test update
 - remve whitespace
 - update
 - Merge branch 'master' into nullStrings
 - first impl

Changes: https://git.openjdk.org/jdk/pull/14083/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14083&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300038
  Stats: 183 lines in 9 files changed: 163 ins; 1 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/14083.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14083/head:pull/14083

PR: https://git.openjdk.org/jdk/pull/14083