http2: Fix client write for trailers on stream close

Check that the trailer buffer exists before attempting a client write
for trailers on stream close.

Refer to comments in https://github.com/bagder/curl/pull/564

lib/http2.c

@@ -1043,6 +1043,7 @@ static ssize_t http2_handle_stream_close(struct connectdata *conn,
     return -1;
   }
 
+  if(stream->trailer_recvbuf && stream->trailer_recvbuf->buffer) {
     trailer_pos = stream->trailer_recvbuf->buffer;
     trailer_end = trailer_pos + stream->trailer_recvbuf->size_used;
 
@@ -1059,6 +1060,7 @@ static ssize_t http2_handle_stream_close(struct connectdata *conn,
 
       trailer_pos += n + 1;
     }
+  }
 
   DEBUGF(infof(data, "http2_recv returns 0, http2_handle_stream_close\n"));
   return 0;