RFR: 8266087: Move 'buffer' declaration in get_user_name_slow() inside of linux specific code

[Harold Seigel]

Please review this very small fix for JDK-8266087.  The fix was tested by running Mach5 tiers 1 and 2 on Linux, Mac OS, and Windows.

Thanks, Harold

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Commit messages:
 - 8266087: Move 'buffer' declaration in get_user_name_slow() inside of linux specific code

Changes: https://git.openjdk.java.net/jdk/pull/3761/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3761&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266087
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3761.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3761/head:pull/3761

PR: https://git.openjdk.java.net/jdk/pull/3761