liblog: tests: crashes in logcat.descriptive

scanTagLine has some loops that do not check for a terminating nul,
problematic for responses from the logger via __getEventTag.

Test: gTest logcat-unit-tests --gtest_filter=*.descriptive
Bug: 37791296
Change-Id: Ib4e900b765b407bdee43e009d6625fe289863ddc