RFR: JDK-8259786: initialize last parameter of getpwuid_r

[Matthias Baesken]

We have a couple of calls to getpwuid_r  in the codebase, like 
g= getpwuid_r(getuid(), &resbuf, pwd_buf, sizeof(pwd_buf), &pwd);

Usually we NULL-check pwd after the call because we do not fully trust the return code of the function (it is documented in the codebase why we do not fully trust the return code) . However we miss to initialize pwd at some places before the call, which might we a little problematic and should be improved   (at other places we already initialize it).

This triggers also Sonar warnings like :
https://sonarcloud.io/project/issues?id=jdk&open=AXaE0dsA8L9hkQskGEbA&resolved=false&types=BUG


Aside from this issue ,  should we in other issue ,  unify the OS versions of  static char* get_user_name(uid_t uid)    in posix code (currently we have it for bsd, linux, aix  but the functions look very similar ?

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Commit messages:
 - JDK-8259786

Changes: https://git.openjdk.java.net/jdk/pull/2098/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2098&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259786
  Stats: 13 lines in 4 files changed: 0 ins; 4 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2098.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2098/head:pull/2098

PR: https://git.openjdk.java.net/jdk/pull/2098