[8u] RFR (S) 8207779: Method::is_valid_method() compares 'this' with NULL

[Aleksey Shipilev]

Original bug:
    https://bugs.openjdk.java.net/browse/JDK-8207779
    https://hg.openjdk.java.net/jdk/jdk/rev/b5aac518b097

I was looking at backporting this to 8u as the pre-requisite for JDK-8231949. I have suggested 
JDK-8231949 without this pre-requisite, which also matches how 11u backported without this fix.

But, I think the fix is actually simple enough to backport and it dodges UB as a good bonus. The 
patch does not apply to 8u cleanly, and requires adjustments to fit the context differences. I 
grepped the source for is_valid_method to catch all other places where this needs to be adjusted.

8u webrev:
    https://cr.openjdk.java.net/~shade/8207779/webrev.8u.01

Testing: Linux x86_64 fastdebug, {tier1, jdk_jfr}

-- 
Thanks,
-Aleksey