discovery: Return error from decodeAdInfo.

The function was not returning an err and causing callers to panic.

Change-Id: Ia119fd7a709d7ee63236afa8ef3d2ba32528fa6f
diff --git a/lib/discovery/plugins/ble/encoding.go b/lib/discovery/plugins/ble/encoding.go
index d76fc95..47c029f 100644
--- a/lib/discovery/plugins/ble/encoding.go
+++ b/lib/discovery/plugins/ble/encoding.go
@@ -213,7 +213,7 @@
 	adinfo.Status = idiscovery.AdStatus(readInt())
 
 	if err != nil {
-		adinfo = nil
+		return nil, err
 	}
 	return adinfo, nil
 }