http2: lazy init header_recvbuf

It makes us use less memory when not doing HTTP/2 and subsequently also
makes us not have to cleanup HTTP/2 related data when not using HTTP/2!

lib/http.c

@@ -163,7 +163,6 @@ CURLcode Curl_http_setup_conn(struct connectdata *conn)
 
   conn->data->req.protop = http;
 
-  http->header_recvbuf = Curl_add_buffer_init();
   http->nread_header_recvbuf = 0;
   http->bodystarted = FALSE;
   http->status_code = -1;

lib/http2.c

@@ -1168,6 +1168,9 @@ CURLcode Curl_http2_setup(struct connectdata *conn)
 
   stream->stream_id = -1;
 
+  if(!stream->header_recvbuf)
+    stream->header_recvbuf = Curl_add_buffer_init();
+
   if((conn->handler == &Curl_handler_http2_ssl) ||
      (conn->handler == &Curl_handler_http2))
     return CURLE_OK; /* already done */