Added thread stack memory leak check to vvp.
Added thread stack memory leak check to vvp.

When a vvp thread terminates, its real and string stacks should be empty.
Adding assertions to this effect catches some code generator bugs.

themperek/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog