if no comparison function set make sk_sort no op

Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/stack/stack.c

@@ -336,7 +336,7 @@ void *sk_set(_STACK *st, int i, void *value)
 
 void sk_sort(_STACK *st)
 {
-    if (st && !st->sorted) {
+    if (st && !st->sorted && st->comp != NULL) {
         int (*comp_func) (const void *, const void *);
 
         /*