Fix use-after-free with unsat cores/proofs (#174)

Fix use-after-free with unsat cores/proofs (#174)

In TSatProof<Solver>::finalizeProof(), we got a clause from the clause
allocator, called resolveUnit() and then size() on the clause. The problem is
that resolveUnit() can reallocate memory (and there is even a comment warning
about that in finalizeProof()), which invalidates the clause. This commit gets
the clause again from the clause allocator before calling size().