Fix OOB access (#3383)

Fix OOB access (#3383)

In theory_engine.cpp, we were calling `theoryOf()` with
`THEORY_SAT_SOLVER` as the theory id. However, `THEORY_SAT_SOLVER` is
defined as `THEORY_LAST` and thus out-of-bounds of the `d_theoryTable`
defined in theory_engine.h (which is of size `THEORY_LAST`. This commit
adds an assertion that detects the out-of-bound access and introduces a
method to turn a theory id into a string which correctly handles
`THEORY_SAT_SOLVER`.