RFR: JDK-8266918: merge_stack in check_code.c add NULL check [v2]

[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 .

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  Replace check by assertion

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3979/files
  - new: https://git.openjdk.java.net/jdk/pull/3979/files/eeff35ef..d58c996e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3979&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3979&range=00-01

  Stats: 2 lines in 1 file changed: 1 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