Fix nil pointer panic in beacon event stream on context cancellation

When subscribeStream returns nil (e.g. context canceled), startStream
now returns early instead of falling through to the select loop where
it would dereference nil basicStream fields.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: barnabasbusa

clients/consensus/rpc/beaconstream.go

@@ -76,9 +76,10 @@ func (bs *BeaconStream) startStream() {
 	// Subscribe to basic events (block, head, finalized_checkpoint)
 	basicEvents := bs.events & (StreamBlockEvent | StreamHeadEvent | StreamFinalizedEvent)
 	basicStream := bs.subscribeStream(bs.client.endpoint, basicEvents)
-	if basicStream != nil {
-		defer basicStream.Close()
+	if basicStream == nil {
+		return
 	}
+	defer basicStream.Close()
 
 	// Subscribe to advanced events (execution_payload_available, execution_payload_bid)
 	// These are in a separate stream because clients may not support them yet,