Incomplete null checks for JNI-refs

[babsingh]

In JVMTI (and few instances outside JVMTI), there are occurrences of incomplete null checks for JNI-refs.

Example:

jobject ref = ...;
...
if (NULL == ref) { /* Incomplete */
   ...

It's legal to have a JNI ref to NULL (think weak global ref).

A complete null check will be as follows:

jobject ref = ...;
...
j9object_t obj = (NULL == ref) ? NULL : J9_JNI_UNWRAP_REFERENCE(ref);
if (NULL == obj) {
   ...

[babsingh]

fyi @gacholio

[gacholio]

There may be other instances outside of JVMTI.

The most readable way of fixing this is to use a local for the unwrapped object:

j9object_t obj = (NULL == ref) ? NULL : J9_JNI_UNWRAP_REFERENCE(ref);
if (NULL == obj) {
}

[gacholio]

Another option would be to handle NULL refs in J9_JNI_UNWRAP_REFERENCE. This would mean removing some existing NULL checks to avoid duplication. The downside is that the NULL checks are sometimes unnecessary (no native will ever be called with a wrapped NULL).