RFR(S): 8167299: -XX:+PrintRelocations crashes the VM

[Volker Simonis]

Hi,

can I please have a review and sponsor for this small fix which
re-enables the usage of -XX:+PrintRelocations:

https://bugs.openjdk.java.net/browse/JDK-8167299
http://cr.openjdk.java.net/~simonis/webrevs/2016/8167299/

If we're running with "PrintNMethods" (which is as debug option) a
call to Relocation::code() in Relocation::method_value() may return
NULL. We must check this and handle it appropriately.

Also, if running with "-XX:+WizardMode",
RelocIterator::print_current() calls oop->print_value_on() without
checking if oop is actually a valid oop. This can be easily fixed by
calling oop->is_oop() before calling oop->print_value_on().

Thank you and best regards,
Volker