ANDROID: xt_quota2: clear quota2_log message before sending

Initialize message buffer for quota2_log to avoid sending
random data.

Bug: 196046570
Test: passed netd test suites
Fixes: 10cda83af99d ("ANDROID: netfilter: xt_quota2: adding the
original quota2 from xtables-addons")
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: Ic9f34aaa2593809b375fc649b74567190c99dc62