Fix crashes in hash table scanning with THREADED_RTS
Fix crashes in hash table scanning with THREADED_RTS

See comments.

olsner/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,shlevy/ghc