With new emails we could have negative UIDs in come use cases. Exclude these
from the list of UIDs. The negative UIDs lead to invalid SEARCH command:
SEARCH command error: BAD ['Could not parse command']. Data: FMAO19 SEARCH (UID -4:*)
Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/512
Tested-by: https://github.com/shubhamkrm
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>