fix raw math query panic

[mengjinglei]

influxDB will panic when apply math query like:
select value + value from cpu
this pr fix this, in addition, this pr make select value + value from cpu return the correct value

[otoolep]

This looks good @mengjinglei -- thanks.

Can you sign the CLA? https://influxdb.com/community/cla.html Once you do that we can get this merged.

[otoolep]

@dgnorton @corylanou

[corylanou]

+1. Great test coverage.

[otoolep]

Waiting for CLA to be signed before merging.

[otoolep]

CLA has already been signed.

[mengjinglei]

sorry for the late reply, this pr only fix math query without aggregate functions like
select value + value from cpu
but can not handle math query with aggregate functions like
select mean(value) + max(value) from cpu or select min(value) + temperature from cpu
I'm still working on this and will send a pr ASAP, thanks for your updating

[otoolep]

Great, thanks again @mengjinglei