Fix length of (*DatabaseIndex).SeriesKeys()
Fix length of (*DatabaseIndex).SeriesKeys()

Previously, it would return as many empty strings in the first half of
the slice as valid values at the end of the slice.

runner-mei/influxdb,nooproblem/influxdb,joaopedrotaveira/influxdb,influxdb/influxdb,influxdb/influxdb,li-ang/influxdb,pkittenis/influxdb,apigee/influxdb,nooproblem/influxdb,apigee/influxdb,influxdb/influxdb,jgeiger/influxdb,mark-rushakoff/influxdb,mnuessler/influxdb,jgeiger/influxdb,chenziliang/influxdb,vladlopes/influxdb,jgeiger/influxdb,runner-mei/influxdb,pkittenis/influxdb,influxdata/influxdb,linearb/influxdb,archsaber/influxdb,joaopedrotaveira/influxdb,pkittenis/influxdb,jgeiger/influxdb,daachi/influxdb,archsaber/influxdb,benbjohnson/influxdb,influxdb/influxdb,nooproblem/influxdb,nooproblem/influxdb,apigee/influxdb,otoolep/influxdb,mark-rushakoff/influxdb,apigee/influxdb,linearb/influxdb,daachi/influxdb,joaopedrotaveira/influxdb,linearb/influxdb,influxdb/influxdb,otoolep/influxdb,otoolep/influxdb,mnuessler/influxdb,daachi/influxdb,mnuessler/influxdb,li-ang/influxdb,vladlopes/influxdb,archsaber/influxdb,chenziliang/influxdb,apigee/influxdb,daachi/influxdb,linearb/influxdb,benbjohnson/influxdb,nooproblem/influxdb,jgeiger/influxdb,joaopedrotaveira/influxdb,runner-mei/influxdb,joaopedrotaveira/influxdb,pkittenis/influxdb,daachi/influxdb,runner-mei/influxdb,chenziliang/influxdb,pkittenis/influxdb,mnuessler/influxdb,mark-rushakoff/influxdb,apigee/influxdb,otoolep/influxdb,archsaber/influxdb,mnuessler/influxdb,daachi/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,linearb/influxdb,chenziliang/influxdb,mark-rushakoff/influxdb,vladlopes/influxdb,mnuessler/influxdb,otoolep/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,benbjohnson/influxdb,benbjohnson/influxdb,influxdb/influxdb,otoolep/influxdb,jgeiger/influxdb,pkittenis/influxdb