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

[coleen.phillimore at oracle.com]

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