Fix broken PPRF

src/pprf/HttpClient.vala

@@ -77,7 +77,7 @@ namespace Pprf {
 
         public class Receipticle : OutputStream {
 
-            private ByteBuffer output;
+            private ByteBuffer output = new ByteBuffer(0);
             private const int max_size = 8192;
 
             private Cond cond = Cond ();