[poll] avoid infinite loop in ssh_poll_ctx_free()

(cherry picked from commit eace59d3eaad413fd846d1c8b4c926285f6cd5ab)

src/poll.c

@@ -495,7 +495,12 @@ void ssh_poll_ctx_free(ssh_poll_ctx ctx) {
 
       /* force poll object removal */
       if (p->cb && p->cb(p, fd, POLLERR, p->cb_data) < 0) {
-        used = ctx->polls_used;
+        if(ctx->polls_used < used) {
+            used = ctx->polls_used;
+        } else {
+            /* nothing to do */
+            i++;
+        }
       } else {
         i++;
       }