stacks: don't call si_set_finalizer

si_set_finalizer is CL-world function and returns 0 values. That means
in particular, that env->nvalues is changed.

In this situation, when new binding was introduced, we could lose our
nvalues, what lead to invalid multiple-value-bind (next commit will
contain a regression test).

We use unprotected version. If interrupts cause problems with it, we
may need to wrap it in disable_interrupts. Threading code uses
ecl_set_finalizer_unprotected without such wrapping though, so I
believe that should be safe.

Fixes #233.

src/c/stacks.d

@@ -274,7 +274,7 @@ ecl_new_binding_index(cl_env_ptr env, cl_object symbol)
     symbol->symbol.binding = new_index;
     symbol->symbol.dynamic |= 1;
   }
-  si_set_finalizer(symbol, ECL_T);
+  ecl_set_finalizer_unprotected(symbol, ECL_T);
   return new_index;
 }