Panic with InfluxQL node

[desa]

Built from: 60e2a07

Using the following tickscript

var s_mean = batch
  |query('SELECT count(val) as cnt_val from "poc"."autogen"."payments"')
    .period(14m)
    .every(10s)
    .groupBy(time(1m))
    .offset(1m)
    .fill(1)
  |mean('cnt_val')
    .as('val')
  |log()

var s_max = batch
    |query('SELECT count(*) as val FROM "poc"."autogen"."payments"')
      .period(1m)
      .every(10s)
      .groupBy(time(1m))
      .fill(1)
    |mean('val')
      .as('val')
    |log()

s_mean
    |join(s_max)
        .as('s_mean', 's_max')

with the following influx-stress script

influx-stress insert --db "poc" -s 100 -b 100 --pps 100 payments,s=1 val=10

yields the following panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xa0 pc=0x290b6a]

goroutine 49 [running]:
panic(0x18d8b00, 0xc42000c0e0)
	/usr/local/go/src/runtime/panic.go:500 +0x1a1
github.com/influxdata/kapacitor.(*InfluxQLNode).runBatchInfluxQL(0xc420486900, 0x0, 0x0)
	/Users/michaeldesa/go/src/github.com/influxdata/kapacitor/influxql.go:190 +0x11da
github.com/influxdata/kapacitor.(*InfluxQLNode).runInfluxQLs(0xc420486900, 0x0, 0x0, 0x0, 0xc42061ff58, 0xc42061ff68)
	/Users/michaeldesa/go/src/github.com/influxdata/kapacitor/influxql.go:45 +0x4c
github.com/influxdata/kapacitor.(*InfluxQLNode).(github.com/influxdata/kapacitor.runInfluxQLs)-fm(0x0, 0x0, 0x0, 0xc42061ff80, 0x0)
	/Users/michaeldesa/go/src/github.com/influxdata/kapacitor/influxql.go:36 +0x48
github.com/influxdata/kapacitor.(*node).start.func1(0xc420486900, 0x0, 0x0, 0x0)
	/Users/michaeldesa/go/src/github.com/influxdata/kapacitor/node.go:140 +0x8e
created by github.com/influxdata/kapacitor.(*node).start
	/Users/michaeldesa/go/src/github.com/influxdata/kapacitor/node.go:141 +0x5d

[nathanielc]

Fixed in #1385