RFR 8207779: Method::is_valid_method() compares 'this' with NULL

[Harold David Seigel]

Hi,

Please review this JDK-12 fix for bug JDK-8207779.  The fix changes 
function is_valid_method() into a static function to avoid comparisons 
between 'this' and NULL and to avoid accessing the validating method 
through the Method being validated.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8207779/webrev/index.html

JBS Bug:  https://bugs.openjdk.java.net/browse/JDK-8207779

This fix was regression tested by running Mach5 tiers 1 and 2 tests and 
builds on Linux-X64, Windows, Solaris Sparc, and Mac OS X, running tiers 
3-5 tests on Linux-x64, and by running JCK-11 Lang and VM tests on 
Linux-x64.

Thanks, Harold