out_es: use snprintf wrapper function for bulk header.
out_es: use snprintf wrapper function for bulk header.

We used fixed char array to construct bulk header.
If long index comes, it needs larger buffer.

out_es uses snprintf wrapper function.
If the buffer is small, the api increases the buffer.

Signed-off-by: Takahiro Yamashita <8b9f03f78cb27351a99f79113f99e589b65190d5@gmail.com>

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