group: fix compile error

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roup.c:361:45: warning: argument to ‘sizeof’ in ‘memcpy’ call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
    memcpy(timestamp, buf + nodes_len, sizeof(timestamp));
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We should pass the real length instead of length of pointer

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>