RFR: 8207025: Simplify PathString::set_value() in arguments.cpp

[Ioi Lam]

Please review this trivial change. Remove unnecessary NULL check and change return type to void, since the function will never return with a failure status.

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Commit messages:
 - 8207025: Simplify PathString::set_value() in arguments.cpp

Changes: https://git.openjdk.java.net/jdk/pull/7981/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7981&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8207025
  Stats: 11 lines in 2 files changed: 1 ins; 6 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7981.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7981/head:pull/7981

PR: https://git.openjdk.java.net/jdk/pull/7981