Fix a crash in requestSync()
Fix a crash in requestSync()

It was possible for a thread to read invalid memory after a conflict
when multiple threads were synchronising.

I haven't been successful in constructing a test case that triggers
this, but we have some internal code that ran into it.

(cherry picked from commit ea3d1efb863ca83b28af9056576d47f1abf98fa9)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc