Runtime: Improved robustness of pop_n_elems().
pop_n_elems() should now survive the stack being used during
the free_mixed_svalues() call. Fixes issue with destruct
callbacks being called during free_mixed_svalues().
Non-freed svalues were overwritten by the stack frame for the
destruct callbacks, and values freed by the destruct callbacks
were freed again, causing fatal error "Using unmarked freed svalue".