Fix panic:runtime error: invalid memory address or nil pointer dereference
Fix panic:runtime error: invalid memory address or nil pointer dereference

github.com/influxdata/influxdb/tsdb.(*Shard).FieldDimensions(0xc820244000, 0xc821b70fb0, 0x1, 0x1, 0xc822b9cc00, 0xc822b9cc30, 0x0, 0x0)
    /Users/jason/go/src/github.com/influxdata/influxdb/tsdb/shard.go:588 +0xa62
github.com/influxdata/influxdb/tsdb.(*shardIteratorCreator).FieldDimensions(0xc8202b6078, 0xc821b70fb0, 0x1, 0x1, 0xc822b9cbd0, 0x0, 0x0, 0x0)
    /Users/jason/go/src/github.com/influxdata/influxdb/tsdb/shard.go:818 +0x53
github.com/influxdata/influxdb/influxql.IteratorCreators.FieldDimensions(0xc821b71250, 0x1, 0x1, 0xc821b70fb0, 0x1, 0x1, 0xc822b9cba0, 0xc822b9cbd0, 0x0, 0x0)
    /Users/jason/go/src/github.com/influxdata/influxdb/influxql/iterator.go:639 +0x15a
github.com/influxdata/influxdb/influxql.(*IteratorCreators).FieldDimensions(0xc822a32ae0, 0xc821b70fb0, 0x1, 0x1, 0x20, 0x18, 0x0, 0x0)
    <autogenerated>:163 +0xd3

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