github.com/cockroachdb/cockroach/pkg/ccl/importccl: _double_null failed under stress

[cockroach-teamcity]

SHA: https://github.com/cockroachdb/cockroach/commits/f818c4c3b946c40839921c72fc1322fb3b385ee6

Parameters:

TAGS=
GOFLAGS=-race

Failed test: https://teamcity.cockroachdb.com/viewLog.html?buildId=772409&tab=buildLog

=== RUN   TestImportData/MYSQLOUTFILE:_double_null
I180711 06:32:13.289772 2136 ccl/importccl/read_import_proc.go:82  [import-distsql,n1] could not fetch file size; falling back to per-file progress: bad ContentLength: -1
I180711 06:32:13.419312 2180 storage/replica_command.go:275  [split,n1,s1,r38/1:/{Table/66-Max}] initiating a split of this range at key /Table/67 [r39]

[knz]

Dup #27360.

[knz]

Also #27498