RFR: 8283287: ClassLoader.c cleanups [v5]

[Tyler Steele]

> As mentioned in the issue, I'd like to perform the following tidying on ClassLoader.c
> 
> - Alphabetize includes.
> - Replace 'if (ptr == 0)' with 'if (ptr == NULL)'.
> - Replace 'return 0' with 'return NULL'.

Tyler Steele has updated the pull request incrementally with one additional commit since the last revision:

  Makes requested readability changes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7846/files
  - new: https://git.openjdk.java.net/jdk/pull/7846/files/2fd5b368..9a8e6b6f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7846&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7846&range=03-04

  Stats: 8 lines in 1 file changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7846.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7846/head:pull/7846

PR: https://git.openjdk.java.net/jdk/pull/7846