RFR: JDK-8303072: Memory leak in exeNullCallerTest.cpp

[Justin King]

Fix trivial memory leak that occurs during tests.

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Commit messages:
 - Memory leak in exeNullCallerTest.cpp

Changes: https://git.openjdk.org/jdk/pull/12715/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303072
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12715.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12715/head:pull/12715

PR: https://git.openjdk.org/jdk/pull/12715