BUG/MINOR: deviceatlas: ignore the scheduler's update in progress marker

BUG/MINOR: deviceatlas: ignore the scheduler's update in progress marker

The scheduler sets the first byte of the shared map to '0' while it
rewrites the data file path. Any non-zero byte was taken as a pending
update, so a request could read a partial path and emit a spurious
"invalid path" alert. '0' is now treated as no update, as the nginx
module does.

This should be backported to lower branches.