http2: on_frame_recv: return early on stream 0

Coverity CID 1299426 warned about possible NULL dereference otherwise,
but that would only ever happen if we get invalid HTTP/2 data with
frames for stream 0. Avoid this risk by returning early when stream 0 is
used.

lib/http2.c

@@ -235,6 +235,9 @@ static int on_frame_recv(nghttp2_session *session, const nghttp2_frame *frame,
     }
     stream = data_s->req.protop;
   }
+  else
+    /* we do nothing on stream zero */
+    return 0;
 
   switch(frame->hd.type) {
   case NGHTTP2_DATA: