lisp stack: deallocate the old stack after resizing

Just like with other stacks we call ecl_dealloc on the old stack. Previously
we had stack frames that could have referenced it, but we can treat it like
other stacks, because there are not lingering references.

src/c/stacks.d

@@ -169,6 +169,7 @@ ecl_stack_set_size(cl_env_ptr env, cl_index tentative_new_size)
   }
   ECL_STACK_RESIZE_ENABLE_INTERRUPTS(env);
 
+  ecl_dealloc(old_stack);
   return env->stack_top;
 }