[core] Be sure we don't leave ctx->current_sub uninitialized. While it *usually* will be init'd shortly after context creation, there's no promise of that, and this uninitialized pointer was being hit and followed during a GC run, causing an assertion failure and/or segfault.
[core] Be sure we don't leave ctx->current_sub uninitialized. While it *usually* will be init'd shortly after context creation, there's no promise of that, and this uninitialized pointer was being hit and followed during a GC run, causing an assertion failure and/or segfault.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@41796 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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