logd: regression in log_strtok_r

In commit 'logd: fix kernel logline stutter'
2c3b300f we introduced log_strtok_r.
as a replacement for strtok_r that dealt with a problem with
some kernel log messages. Fix is to refine definition of
is_timestamp to not match on patterns like [0].

Change-Id: I0867a555a3bca09bbf18d18e75e41dffffe57a22