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.
Handle out-of range and undefined LHS word indices in assignments.

For constant word indices, issue a warning if the index is out of
range or an undefined value. In any case, the RHS value should be
discarded, and the actual assignment should be skipped.
Handle out-of-bounds and undefined word indices in constant functions.
Fix vvp memory leak for VPI call with no calltf.

If a VPI call with real arguments has no calltf function, we still
need to pop the arguments off the vthread stack. Similarly, if it
has a real result, we need to push a value onto the vthread stack.