RFR: JDK-8301223: Replace NULL with nullptr in share/gc/g1/ [v3]

[Johan Sjölen]

> Hi, this PR changes all occurrences of NULL to nullptr for the subdirectory share/gc/g1/. Unfortunately the script that does the change isn't perfect, and so we
> need to comb through these manually to make sure nothing has gone wrong. I also review these changes but things slip past my eyes sometimes.
> 
> Here are some typical things to look out for:
> 
> 1. No changes but copyright header changed (probably because I reverted some changes but forgot the copyright).
> 2. Macros having their NULL changed to nullptr, these are added to the script when I find them. They should be NULL.
> 3. nullptr in comments and logs. We try to use lower case "null" in these cases as it reads better. An exception is made when code expressions are in a comment.
> 
> An example of this:
> 
> ```c++
> // This function returns null
> void* ret_null();
> // This function returns true if *x == nullptr
> bool is_nullptr(void** x);
> 
> 
> Note how `nullptr` participates in a code expression here, we really are talking about the specific value `nullptr`.
> 
> Thanks!

Johan Sjölen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge remote-tracking branch 'origin/master' into JDK-8301223
 - Last NULL
 - Merge remote-tracking branch 'origin/master' into JDK-8301223
 - Missed fix
 - Fixes
 - Merge remote-tracking branch 'origin/master' into JDK-8301223
 -  Replace NULL with nullptr in share/gc/g1/

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

Changes: https://git.openjdk.org/jdk/pull/12248/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12248&range=02
  Stats: 808 lines in 83 files changed: 0 ins; 0 del; 808 mod
  Patch: https://git.openjdk.org/jdk/pull/12248.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12248/head:pull/12248

PR: https://git.openjdk.org/jdk/pull/12248