Fix ABC build (#2808)

PR #2786 introduced a pure virtual method `TBitblaster::getSatSolver()`.
`AigBitblaster` was missing the implementation of that method. This
commit adds an implementation that simply returns the underlying SAT
solver. Note: The method is currently only used for proofs and CVC4 does
not support proofs in combination with ABC. To make this explicit, the
commit also adds a check in `SmtEngine::setDefaults()` that makes sure
that we are not trying to produce proofs with `--bitblast-aig` (before
the commit, we just crashed with an assertion failure/null pointer
dereference).

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