tftp-hpa: bug fix on empty file put error

With the feature that checking the disk filled up, the return value of
function write_behind was checked and used to detect the disk status.
While for empty file, without data being written, this function will
return -1 thus the disk filled up error was miss-raised. Fix it.

Signed-off-by: Zhang Xiao <xiao.zhang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>