RFR: 8305650: os::lasterror incorrectly truncates buffer in os_windows.cpp

[Julian Waters]

n in os::lasterror defined by os_windows.cpp is only the length of the string's actual character data, not accounting for the null character terminating the string. We should terminate the n + 1 character, not the n'th character

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Commit messages:
 - os::lasterror truncation error in os_windows.cpp

Changes: https://git.openjdk.org/jdk/pull/13350/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305650
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13350.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13350/head:pull/13350

PR: https://git.openjdk.org/jdk/pull/13350