protect the code from nil map in GetRemoteDoc()

_time=2019-05-03T17:26:07.869+00:00 _level=SEVERE _msg=panic: assignment to entry in nil map

request text:

<ud>select prepareds.* from system:prepareds</ud>

stack:

goroutine 22042215 [running]:

github.com/couchbase/query/execution.(*Context).Recover(0xc43e4e5a20)

        goproj/src/github.com/couchbase/query/execution/context.go:522 +0xbc

panic(0xe3cd20, 0xc43d4f1120)

        /home/couchbase/.cbdepscache/exploded/x86_64/go-1.8.5/go/src/runtime/panic.go:489 +0x2cf

github.com/couchbase/query/value.objectValue.SetField(0x0, 0xfa3ef4, 0x4, 0xdf0180, 0xc43d4f1110, 0xc53ee8da18, 0xc43d4f1110)

        goproj/src/github.com/couchbase/query/value/object.go:251 +0xd1

github.com/couchbase/query/value.(*annotatedValue).SetField(0xc5bc4e0fc0, 0xfa3ef4, 0x4, 0xdf0180, 0xc43d4f1110, 0xc53ee8d9f0, 0x5471d3)

        <autogenerated>:89 +0x78

github.com/couchbase/query/datastore/system.(*preparedsKeyspace).Fetch.func1(0x0)

        goproj/src/github.com/couchbase/query/datastore/system/system_keyspace_prepareds.go:87 +0x190

github.com/couchbase/query/server/http.(*systemRemoteHttp).GetRemoteDoc(0xc42008e740, 0xc448f7c9e0, 0x12, 0xc5bc4e04e0, 0x24, 0xfa99dc, 0x9, 0xfa3a6c, 0x4, 0xc5bc4e0510, ...)

        goproj/src/github.com/couchbase/query/server/http/system_remote.go:257 +0x323

github.com/couchbase/query/datastore/system.(*preparedsKeyspace).Fetch(0xc4206aea20, 0xc5d0943000, 0x10, 0x40, 0xc5761fd440, 0x18a24a0, 0xc43e4e5a20, 0x0, 0x0, 0x0, ...)

        goproj/src/github.com/couchbase/query/datastore/system/system_keyspace_prepareds.go:96 +0x322

github.com/couchbase/query/execution.(*Fetch).flushBatch(0xc43d4d54a0, 0xc43e4e5a20, 0x190e600)

        goproj/src/github.com/couchbase/query/execution/fetch.go:115 +0x533

github.com/couchbase/query/execution.(*base).enbatchSize(0xc43d4d54a0, 0x18d9280, 0xc5d51a38c0, 0x18a2500, 0xc43d4d54a0, 0x10, 0xc43e4e5a20, 0xc5d51a38c0)

        goproj/src/github.com/couchbase/query/execution/base.go:707 +0x1a2

github.com/couchbase/query/execution.(*Fetch).processItem(0xc43d4d54a0, 0x18d9280, 0xc5d51a38c0, 0xc43e4e5a20, 0x1)

        goproj/src/github.com/couchbase/query/execution/fetch.go:60 +0x6a

github.com/couchbase/query/execution.(*base).runConsumer.func1()

        goproj/src/github.com/couchbase/query/execution/base.go:547 +0x25d

github.com/couchbase/query/util.(*Once).Do(0xc43d4d5598, 0xc43b0b3738)

        goproj/src/github.com/couchbase/query/util/sync.go:51 +0x68

github.com/couchbase/query/execution.(*base).runConsumer(0xc43d4d54a0, 0x189fc40, 0xc43d4d54a0, 0xc43e4e5a20, 0x0, 0x0)

        goproj/src/github.com/couchbase/query/execution/base.go:552 +0xaf

github.com/couchbase/query/execution.(*Fetch).RunOnce(0xc43d4d54a0, 0xc43e4e5a20, 0x0, 0x0)

        goproj/src/github.com/couchbase/query/execution/fetch.go:56 +0x5c

created by github.com/couchbase/query/execution.(*base).runConsumer.func1

        goproj/src/github.com/couchbase/query/execution/base.go:537 +0x2f6