pipe: remove useless assertion (#3302)

This assertion was added when req->write_buffer was a pointer. It was
then checking that write_buffer itself was not NULL. Checking that .base
is not NULL is superfluous because WriteFile will return error 998
(ERROR_NO_ACCESS) if the input buffer is invalid. This assertion fires
on zero-length writes when base==NULL&&len==0.

src/win/pipe.c

@@ -1252,7 +1252,6 @@ static DWORD WINAPI uv_pipe_writefile_thread_proc(void* parameter) {
   assert(req != NULL);
   assert(req->type == UV_WRITE);
   assert(handle->type == UV_NAMED_PIPE);
-  assert(req->write_buffer.base);
 
   result = WriteFile(handle->handle,
                      req->write_buffer.base,