simulator: add failsafe for stochastic guards

Stochastic guards are supposed to be generated in a way that the if
branches are disjoint and exhaustive. Compilers have trouble verifying
the latter property, as they usually do not pick up postconditions on
return value of myrand(0, 99).

To silence warnings regarded control paths that do not return a value,
and to add hardening against code generation bugs that actually violate
exhaustion of the if branches, assert() and abort() at the end of such
functions.