RFR: JDK-8266918: merge_stack in check_code.c add NULL check

[Matthias Baesken]

Hello, please review this small Sonar finding.
Sonar reports a potential NULL pointer dereference here :
https://sonarcloud.io/project/issues?id=shipilev_jdk&languages=c&open=AXck8CPLBBG2CXpcnh_z&resolved=false&severities=MAJOR&types=BUG
"Access to field 'item' results in a dereference of a null pointer (loaded from variable 'new')"
It would be better to add a check .

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Commit messages:
 - JDK-8266918

Changes: https://git.openjdk.java.net/jdk/pull/3979/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3979&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266918
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3979.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3979/head:pull/3979

PR: https://git.openjdk.java.net/jdk/pull/3979