[PMC] Fixed RetContinuation recycling in invoke() by invalidating the from context's pointer to the RetCont PMC.  This fixes an assert in an unoptimized build in PGE, created by r42058.  This also requires that the RetCont's from_ctx pointer can't point to the same place as its to_ctx, or else the wrong context gets nulled out.
[PMC] Fixed RetContinuation recycling in invoke() by invalidating the from
context's pointer to the RetCont PMC.  This fixes an assert in an unoptimized
build in PGE, created by r42058.  This also requires that the RetCont's
from_ctx pointer can't point to the same place as its to_ctx, or else the wrong
context gets nulled out.

Something may not set the from_ctx pointer correctly before invoking the
RetCont.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@42082 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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