Z3_solver_set_initial_value unhappy if called before any assertions

[jberdine]

I'm not sure what the desired change is, but Z3_solver_set_initial_value can crash if called before anything is asserted to the solver. For contrast, it looks like cmd_context::set_initial_value builds a set of pending assignments in this case.

See test_solver_set_initial_value.c.txt for a small repro.

This is easy to work around, so FWIW definitely not blocking anything.