RFR: 8189633: Missing -Xcheck:jni checking for DeleteWeakGlobalRef

[Kim Barrett]

> On Aug 21, 2019, at 7:14 PM, Daniel D. Daugherty <daniel.daugherty at oracle.com> wrote:
> 
> On 8/19/19 9:10 PM, Kim Barrett wrote:
> 
> src/hotspot/share/prims/jniCheck.cpp
>     L1931:       if (ref && !JNIHandles::is_weak_global_handle(ref)) {
>         Possible implied boolean. Perhaps:
> 
>                  if (ref != NULL && !JNIHandles::is_weak_global_handle(ref)) {
> 
> Thumbs up! Your call on whether to fix the possible implied boolean.
> No need for another webrev if you do change it.

I used an implicit boolean for cultural compatibility with nearby code.  That new block is
cribbed from the DeleteGlobalRef variant.

Thanks for reviewing.