InfluxDB: Fix corner case where index is too large in ALIAS field

[gabor]

in InfluxDB, when you have a query like this:
SELECT value from "computer.cpu", when the result is drawn on the screen, the time-series will be named "computer.cpu".
you can change this by using the ALIAS field in the query, there are many formats supported by the ALIAS field, but there is one where you use numbers, like $0 or [[1]]. in this case the following will happen:

• we take the measurement name (in this case computer.cpu)
• split it into parts by the dot-character, so we get the array ["computer","cpu"]
• and then use the item from the array at the index specified by the alias (so, in our case, if you use the alias AL $1, the timeseries-name will be AL cpu

there is a problem, if you specify an index that is higher than the length of the array, the series will be named undefined. for example, if your measurement is named computer.cpu and you use the alias AL $7, the series will be named AL undefined.

this pull request changes this so that the series name will become AL $7 in this case. this is consistent with how it happens in the influxdb-alerting-go-code, and also consistent with the approach that when we have a variable that does not exist, we just keep the original text.

how to test:
you need to have measurement with names that contain periods. the simplest way is to manually insert such values into influxdb:

• make devenv sources=influxdb
• run docker ps, and find the ID of the influxdb container.
• run docker exec -it <docker_container_id> influx write -b mybucket -o myorg -t mytoken 'name.with.dots v=40'
• now go to explore-mode, choose the datasource gdev-influxdb-influxql, and choose the measurement "name.with.dots", choose the field "v", and try out ALIAS values.

[Elfo404]

LGTM!

do we want to add type/bug and add to changelog labels?