RFR: 8283225: ClassLoader.c produces incorrect OutOfMemory Exception when length is 0 (aix) [v4]

[Tyler Steele]

> As described in the linked issue, NullClassBytesTest fails due an OutOfMemoryError produced on AIX when the test calls defineClass with a byte array of size of 0. The native implementation of defineClass then calls  malloc with a size of 0. On AIX malloc(0) returns NULL, while on other platforms it return a valid address. When NULL is produced by malloc for this reason, ClassLoader.c incorrectly interprets this as a failure due to a lack of memory.
> 
> ~~This PR modifies ClassLoader.c to produce an OutOfMemoryError only when `errno == ENOMEM` and to produce a ClassFormatError with the message "ClassLoader internal allocation failure" in all other cases (in which malloc returns NULL).~~ [edit: The above no longer describes the PR's proposed fix. See discussion below]
> 
> In addition, I performed some minor tidy-up work in ClassLoader.c by changing instances of `return 0` to `return NULL`, and `if (some_ptr == 0)` to `if (some_ptr == NULL)`. This was done to improve the clarity of the code in ClassLoader.c, but didn't feel worthy of opening a separate issue.
> 
> ### Alternatives
> 
> It would be possible to address this failure by modifying the test to accept the OutOfMemoryError on AIX. I thought it was a better solution to modify ClassLoader.c to produce an OutOfMemoryError only when the system is actually out of memory.
> 
> ### Testing
> 
> This change has been tested on AIX and Linux/x86.

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

  Improve comment
  
  - Reword to avoid double use of malloc(X)
  - Remove bug id

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7829/files
  - new: https://git.openjdk.java.net/jdk/pull/7829/files/f05c8d85..f0e2800e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7829&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7829&range=02-03

  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7829.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7829/head:pull/7829

PR: https://git.openjdk.java.net/jdk/pull/7829