sensors: don't mess with -1 in FD_SET/FD_ISSET

DASH seems to have a tendency to pass -1 to FD_SET/FD_ISSET for disabled
sensors.  Bionic will no longer silently drop these, and now aborts upon
encountering them.

Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Merge pull request #26 from courtc/master

sensors: don't mess with -1 in FD_SET/FD_ISSET