Fix use-after-free in eager bitblaster (#1772)

Fix use-after-free in eager bitblaster (#1772)

There was a use-after-free in the eager bitblaster: the context used by
the SAT solver was destroyed before the solver. This lead to a
use-after-free in the destructor of the SAT solver when destroying
context-dependent objects. This commit fixes the issue by changing the
desctruction order such that the context is destroyed after the SAT
solver.

Note: This issue was introduced in commit
a917cc2ab4956b542b1f565abf0e62b197692f8d because d_nullContext and
d_satSolver were changed to be std::unique_ptrs.