RFR (trivial) 8210861: Move assert to help diagnose rare RedefineStress crash

[Lois Foltan]

Looks good & trivial.
Lois

On 9/18/2018 12:10 PM, coleen.phillimore at oracle.com wrote:
> Summary: assert that Method being marked on stack hasn't been missed 
> by previous metadata walk
>
> Ran hs-tier1-7 with no failures, which is unfortunate because this 
> assert will tell me where the Method was missed from previous walk.
>
> open webrev at http://cr.openjdk.java.net/~coleenp/8210861.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8210861
>
> Thanks,
> Coleen