Pin expectMsgId after KEXDH_GEX_GROUP

- Set expectMsgId to MSGID_KEXDH_GEX_INIT so the server rejects
  any other KEX message at this stage (RFC 4419 sec 3).
- Reject NULL ssh->handshake in the entry check now that the
  success path dereferences it.

Issue: F-3668

Author: ejohnstown

src/internal.c

@@ -13478,7 +13478,7 @@ int SendKexDhGexGroup(WOLFSSH* ssh)
     int ret = WS_SUCCESS;
 
     WLOG(WS_LOG_DEBUG, "Entering SendKexDhGexGroup()");
-    if (ssh == NULL)
+    if (ssh == NULL || ssh->handshake == NULL)
         ret = WS_BAD_ARGUMENT;
 
     if (ret == WS_SUCCESS) {
@@ -13525,8 +13525,11 @@ int SendKexDhGexGroup(WOLFSSH* ssh)
         ret = BundlePacket(ssh);
     }
 
-    if (ret == WS_SUCCESS)
+    if (ret == WS_SUCCESS) {
+        WLOG_EXPECT_MSGID(MSGID_KEXDH_GEX_INIT);
+        ssh->handshake->expectMsgId = MSGID_KEXDH_GEX_INIT;
         ret = wolfSSH_SendPacket(ssh);
+    }
 
     WLOG(WS_LOG_DEBUG, "Leaving SendKexDhGexGroup(), ret = %d", ret);
     return ret;