Fix incorrect port of write_to_fd

fish-rust/src/wutil/mod.rs

@@ -457,6 +457,7 @@ fn do_write(fd: RawFd, total_written: &mut usize, mut buf: &[u8]) -> bool {
             unsafe {
                 std::ptr::copy(&buff[0], &mut accum[accumlen], buff.len());
             }
+            accumlen += buff.len();
             true
         } else {
             // Too much data to even fit, just write it immediately.