RFR 8146984: SIGBUS: bool Method::has_method_vptr(const void*)+0xc

[Coleen Phillimore]

Summary: Add address check and use SafeFetchN for Method* vptr access 
when Method* may be bad pointer.

Tested with RBT and failing test case (reproduced 1 in 100 times) with 
fatal in the 'return's in the change to verify.

open webrev at http://cr.openjdk.java.net/~coleenp/8146984/
bug link https://bugs.openjdk.java.net/browse/JDK-8146984

Thanks,
Coleen