Fixes memory leak when an exception goes through runCvc4(). (Fixes #4590) (#4750)

Fixes memory leak when an exception goes through runCvc4(). (Fixes #4590) (#4750)

As noted in #4590, CVC4 may leak memory if an exception is thrown that interrupts the command execution loop in runCvc4().
While not a major issue (the binary is terminated anyway in this case, this is also why we label it as feature), we should fix it nevertheless.

This commit makes sure that the current command is properly managed by using `std::unique_ptr` to handle its deletion.