RFR: JDK-8292586: simplify cleanups in NTLMAuthSequence getCredentialsHandle

[Matthias Baesken]

NTLMAuthSequence getCredentialsHandle has a number of cleanup paths doing various JNU_ReleaseStringPlatformChars calls; those could be placed at one place in the function.

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Commit messages:
 - JDK-8292586

Changes: https://git.openjdk.org/jdk/pull/9914/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292586
  Stats: 18 lines in 1 file changed: 1 ins; 12 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/9914.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9914/head:pull/9914

PR: https://git.openjdk.org/jdk/pull/9914