TimeCheck: fix use of uninitialized variable

Variable 'tag' in TimeCheck::TimeCheckThread::threadLoop
may end up being used w/o prior initialization.
Found by clang-tidy.

Bug: 152526318
Test: m
Change-Id: Ifc1efbee39007ea9cfba8083edbfdb763eafb8dc
Merged-In: Ifc1efbee39007ea9cfba8083edbfdb763eafb8dc