agent: null check ssh->agent in worker

- wolfSSH_AGENT_worker() dereferenced ssh->agent to set the DONE state
  without checking it, so a call before the agent was set up took a null
  dereference.
- Return WS_AGENT_NULL_E instead.

Issue: F-7214

src/agent.c

@@ -1701,6 +1701,11 @@ int wolfSSH_AGENT_worker(WOLFSSH* ssh)
     if (ssh == NULL)
         ret = WS_SSH_NULL_E;
 
+    if (ret == WS_SUCCESS) {
+        if (ssh->agent == NULL)
+            ret = WS_AGENT_NULL_E;
+    }
+
     if (ret == WS_SUCCESS) {
         SendSuccess(NULL);
         DoMessage(NULL, NULL, 0, NULL);