[Fuchsia] Fix incoming_transfer empty validation.

Fix a bug where empty incoming_transfer is not checked that would
cause cast_runner to panic.

Test: Manual test, passing an empty message port on control port would
not cause crash.
Bug: b/145814982

Change-Id: Icdeee7ad3f829e8ac07a08e8b544f364d106e916
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1955089Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722695}

fuchsia/runners/cast/named_message_port_connector.cc

@@ -94,9 +94,11 @@ void NamedMessagePortConnector::OnConnectRequest(
     return;
   }
 
-  if (message.incoming_transfer().size() != 1) {
-    LOG(ERROR) << "Expected one Transferable, got "
-               << message.incoming_transfer().size() << " instead.";
+  size_t num_transfers = message.has_incoming_transfer() ?
+    message.incoming_transfer().size() : 0U;
+  if (num_transfers != 1) {
+    LOG(ERROR) << "Expected one Transferable, got " << num_transfers
+               << " instead.";
     control_port_.Unbind();
     return;
   }