tftp-hpa: fix the empty file writing error

make write_behind to return 0 if a empty file is written, to
distinguish a true writing failure[on which the write_behind
will return -1], then the annoying wrong log will disappear.
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    |Error code 3: Disk full or allocation exceeded
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Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>