RFR: 8265757: stack-use-after-scope in perfMemory_posix.cpp get_user_name_slow()

[Harold Seigel]

Please review this small change to fix JDK-8265757.  The fix was tested with Mach5 tiers 1 and 2 on Linux, MacOS aarch64, and Windows.

Thanks, Harold

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Commit messages:
 - 8265757: stack-use-after-scope in perfMemory_posix.cpp get_user_name_slow()

Changes: https://git.openjdk.java.net/jdk/pull/3630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3630&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265757
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3630.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3630/head:pull/3630

PR: https://git.openjdk.java.net/jdk/pull/3630