Fixed handling of ngx_write_fd() and ngx_read_fd() errors. The ngx_write_fd() and ngx_read_fd() functions return -1 in case of error, so the incorrect comparison with NGX_FILE_ERROR (which is 0 on windows platforms) might result in inaccurate error message in the error log. Also the ngx_errno global variable is being set only if the returned value is -1.

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