RFR (S): 7167254: Crash on OSX in Enumerator.nextElement() with compressed oops

[Roland Westrelin]

One more webrev:
http://cr.openjdk.java.net/~roland/7167254/webrev.02/

I forgot to cast the MacroAssembler::needs_explicit_null_check() argument to intptr_t rather than int.

Roland.