migration/rdma: fix potential nullptr access in rdma_start_incoming_migration

'rdma' is NULL when taking the first error branch in rdma_start_incoming_migration.
And it will cause a null pointer access in label 'err'. Fix that.

Fixes: 59c59c67ee
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200508100755.7875-2-pannengyuan@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Note this is CID 1428762

migration/rdma.c

@@ -4056,7 +4056,9 @@ void rdma_start_incoming_migration(const char *host_port, Error **errp)
     return;
 err:
     error_propagate(errp, local_err);
-    g_free(rdma->host);
+    if (rdma) {
+        g_free(rdma->host);
+    }
     g_free(rdma);
     g_free(rdma_return_path);
 }