[jdk11u-dev] RFR: 8280004: DCmdArgument<jlong>::parse_value() should handle NULL input

[Goetz Lindenmaier]

A low risk fix for diagnostics.

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Commit messages:
 - Backport 55f180fb7dfecc859e59c7852d48aeab6c936a5d

Changes: https://git.openjdk.org/jdk11u-dev/pull/2153/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2153&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280004
  Stats: 11 lines in 1 file changed: 4 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2153.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2153/head:pull/2153

PR: https://git.openjdk.org/jdk11u-dev/pull/2153