Handle nil MeasurementIterator.

[benbjohnson]

@jwilder This should fix the panic you reported along with some additional checks elsewhere.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x999995]

goroutine 45102 [running]:
github.com/influxdata/influxdb/tsdb/index/tsi1.(*FileSet).MeasurementNamesByExpr(0xc4206ea910, 0x0, 0x0, 0x0, 0xc420a99ce0, 0x42b40f, 0xd143c8, 0xc420a99cf0)
    /root/go/src/github.com/influxdata/influxdb/tsdb/index/tsi1/file_set.go:528 +0x65
github.com/influxdata/influxdb/tsdb/index/tsi1.(*Index).MeasurementNamesByExpr(0xc420a7e1e0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /root/go/src/github.com/influxdata/influxdb/tsdb/index/tsi1/index.go:405 +0xb2
github.com/influxdata/influxdb/tsdb/engine/tsm1.(*Engine).MeasurementNamesByExpr(0xc4200aac60, 0x0, 0x0, 0x0, 0x0, 0xc420a99e68, 0x0, 0xffffffffffffffff)
    /root/go/src/github.com/influxdata/influxdb/tsdb/engine/tsm1/engine.go:331 +0x52
github.com/influxdata/influxdb/tsdb.(*Shard).MeasurementNamesByExpr(0xc4206861a0, 0x0, 0x0, 0xc420a99e68, 0xc420a99e38, 0xc4207aaf20, 0x0, 0x1)
    /root/go/src/github.com/influxdata/influxdb/tsdb/shard.go:745 +0x9e
github.com/influxdata/influxdb/tsdb.(*Store).MeasurementNames(0xc4205e5b80, 0xcbde71, 0x3, 0x0, 0x0, 0xc4207aaf20, 0x1, 0x1, 0x0, 0xc420a99f88)
    /root/go/src/github.com/influxdata/influxdb/tsdb/store.go:976 +0x18e
github.com/influxdata/influxdb/tests.TestConcurrentServer_ShowMeasurements.func2()
    /root/go/src/github.com/influxdata/influxdb/tests/server_concurrent_test.go:123 +0x9b
github.com/influxdata/influxdb/tests.runTest.func2(0xc42012f110, 0xc420bc49c0, 0xc4203710e0)
    /root/go/src/github.com/influxdata/influxdb/tests/server_concurrent_test.go:150 +0x7d
created by github.com/influxdata/influxdb/tests.runTest
    /root/go/src/github.com/influxdata/influxdb/tests/server_concurrent_test.go:153 +0xf5

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated