handles.mac:  Fix NewHandle( -1 ) tests
handles.mac:  Fix DisposeHandle( NULL ) test
macos:  Set MemErr to nilHandleErr more often
handles.mac:  Test EmptyHandle( NULL )
handles.mac:  Test GetHandleSize( NULL ) MemErr
Merge branch 'fix-Handle-tests'
v68k-alloc:  Fix multiple-block deallocation

Return the native address of the first block from deallocate_existing(),
not the last.