audio/mac: unbreak build with Clang 6 (C++14 by default)

Console.cpp:36:3: error: cannot initialize a member subobject of type 'int' with an rvalue of type 'nullptr_t'

NULL
^~~~

/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL nullptr

^~~~~~~

Console.cpp:36:3: warning: suggest braces around initialization of subobject [-Wmissing-braces]

NULL
^~~~
{   }

/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL nullptr

^~~~~~~

Console.cpp:36:3: warning: suggest braces around initialization of subobject [-Wmissing-braces]

NULL
^~~~
{   }

/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL nullptr

^~~~~~~

Reported by: antoine (via bug 224669)