Fails with unlimited facets

Trying to use a facet with no hard limit gives the following fatal error:

PDOException: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '-1 OFFSET 0' at line 2: SELECT t_2.value AS value, COUNT(DISTINCT t.item_id) AS num FROM {db_temporary_0} t INNER JOIN {search_api_db_nodes_field_state_name} t_2 ON t.item_id = t_2.item_id GROUP BY value ORDER BY num DESC LIMIT -1 OFFSET 0; Array ( ) in SearchApiDbService->getFacets() (line 1123 of sites/all/modules/search_api_db/service.inc).

The FacetAPI uses -1 to indicate no limit. We should check for this in our service.