c-hyper: don't write to set.writeheader if null

Previously if a caller set CURLOPT_WRITEFUNCTION but did not set a
CURLOPT_HEADERDATA buffer, Hyper would still attempt to write headers
to the data->set.writeheader header buffer, even though it is null.
This led to NPE segfaults attempting to use libcurl+Hyper with Git,
for example.

Instead, process the client write for the status line using the same
logic we use to process the client write for the later HTTP headers,
which contains the appropriate guard logic. As a side benefit,
data->set.writeheader is now only read in one file instead of two.

Fixes curl#6619.
Fixes rustls/rustls-ffi#49.
Fixes hyperium/hyper#2438.

lib/c-hyper.c

@@ -232,12 +232,12 @@ static CURLcode status_line(struct Curl_easy *data,
   len = Curl_dyn_len(&data->state.headerb);
   Curl_debug(data, CURLINFO_HEADER_IN, Curl_dyn_ptr(&data->state.headerb),
              len);
-  Curl_set_in_callback(data, true);
-  wrote = writeheader(Curl_dyn_ptr(&data->state.headerb), 1, len,
-                      data->set.writeheader);
-  Curl_set_in_callback(data, false);
-  if(wrote != len)
-    return CURLE_WRITE_ERROR;
+  result = Curl_client_write(data, CLIENTWRITE_HEADER,
+                             Curl_dyn_ptr(&data->state.headerb), len);
+  if(result) {
+    data->state.hresult = CURLE_ABORTED_BY_CALLBACK;
+    return HYPER_ITER_BREAK;
+  }
 
   data->info.header_size += (long)len;
   data->req.headerbytecount += (long)len;