review (XS) for 6968336: VM crash guarantee(!nm->is_zombie()) failed: cannot lock a zombie method

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/6968336

6968336: VM crash guarantee(!nm->is_zombie()) failed: cannot lock a zombie method
Reviewed-by:

The fix for 6965671 iterates over the code cache and processes
nmethods for which is_alive is true.  It includes an nmethodLocker to
protect the nmethod but it's illegal to lock zombie nmethods.  The
nmethodLocker should be inside the is_alive test instead of outside to
guard against this case.  There was also a ShouldNotReachHere that I
added during testing that I should have removed before putback.
Tested with the jvmti tests under ute.