Access slice only when it has enough bytes

stack:

goroutine 26303247 [running]:

github.com/couchbase/query/execution.(*Context).Recover(0xc4c12bb4a0)

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

panic(0xe3cc00, 0x1875170)

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

github.com/couchbase/query/datastore/couchbase.doFetch(0xc4b8833591, 0x3d, 0xc497e32f60, 0xed45abda4, 0xc430b0ae02)

        goproj/src/github.com/couchbase/query/datastore/couchbase/couchbase.go:1063 +0x554

github.com/couchbase/query/datastore/couchbase.(*keyspace).Fetch(0xc4206d9810, 0xc4a5110000, 0x1, 0x40, 0xc4568eea80, 0x18a24a0, 0xc4c12bb4a0, 0x0, 0x0, 0x0, ...)

        goproj/src/github.com/couchbase/query/datastore/couchbase/couchbase.go:1037 +0x779

github.com/couchbase/query/execution.(*Fetch).flushBatch(0xc47cf614a0, 0xc4c12bb4a0, 0x100000000af0a00)

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

github.com/couchbase/query/execution.(*Fetch).afterItems(0xc47cf614a0, 0xc4c12bb4a0)

        goproj/src/github.com/couchbase/query/execution/fetch.go:72 +0x35

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

        goproj/src/github.com/couchbase/query/execution/base.go:551 +0x296

github.com/couchbase/query/util.(*Once).Do(0xc47cf61598, 0xc475b10f38)

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

github.com/couchbase/query/execution.(*base).runConsumer(0xc47cf614a0, 0x189fc40, 0xc47cf614a0, 0xc4c12bb4a0, 0x0, 0x0)

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

github.com/couchbase/query/execution.(*Fetch).RunOnce(0xc47cf614a0, 0xc4c12bb4a0, 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