hg: panama/dev: 8219119: Need API to create a null Callback

[sundararajan.athijegannathan at oracle.com]

Changeset: 3b115bbe58ae
Author:    sundar
Date:      2019-02-18 16:42 +0530
URL:       http://hg.openjdk.java.net/panama/dev/rev/3b115bbe58ae

8219119: Need API to create a null Callback
Reviewed-by: mcimadamore

! doc/panama_foreign.html
! doc/panama_foreign.md
! src/java.base/share/classes/java/foreign/memory/Callback.java
! src/java.base/share/classes/java/foreign/memory/Pointer.java
! src/java.base/share/classes/jdk/internal/foreign/abi/DirectSignatureShuffler.java
! src/java.base/share/classes/jdk/internal/foreign/abi/UniversalNativeInvoker.java
! src/java.base/share/classes/jdk/internal/foreign/memory/BoundedPointer.java
! src/java.base/share/classes/jdk/internal/foreign/memory/CallbackImpl.java
! src/java.base/share/classes/jdk/internal/foreign/memory/References.java
! test/jdk/com/sun/tools/jextract/jclang-ffi/src/jdk/internal/clang/Cursor.java
! test/jdk/com/sun/tools/jextract/jclang-ffi/src/jdk/internal/clang/Index.java
! test/jdk/com/sun/tools/jextract/libproc/LibprocTest.java
! test/jdk/java/foreign/Upcall/Upcall.java
! test/jdk/java/foreign/Upcall/libUpcall.c
! test/jdk/java/foreign/printf/Printf.java
! test/jdk/java/foreign/types/PointerScopeTest.java
! test/jdk/java/foreign/types/PointerTest.java