panic when right side of LIKE is depends on field

_time=2017-08-11T09:11:35.246-07:00 _level=SEVERE _msg=panic: runtime error: invalid memory address or nil pointer dereference

request text:

select meta().id from default where k0 like k2;

stack:

goroutine 409 [running]:

github.com/couchbase/query/execution.(*Context).Recover(0xc420fea000)

    /Users/xx/master/query/src/github.com/couchbase/query/execution/context.go:477 +0xbc

panic(0x4950220, 0x5107db0)

    /usr/local/Cellar/go/1.8.3/libexec/src/runtime/panic.go:489 +0x2cf

github.com/couchbase/query/expression.(*Identifier).Evaluate(0xc420e1f440, 0x0, 0x0, 0x50a1c00, 0xc420fea000, 0x58, 0x0, 0xc420f59500, 0xc42002fc00)

    /Users/xx/master/query/src/github.com/couchbase/query/expression/identifier.go:49 +0x2b

github.com/couchbase/query/expression.(*BinaryFunctionBase).BinaryEval(0xc42015ba90, 0x5097480, 0xc42015ba90, 0x0, 0x0, 0x50a1c00, 0xc420fea000, 0x4056660, 0x449c7bd, 0x513eaa0, ...)

    /Users/xx/master/query/src/github.com/couchbase/query/expression/function.go:390 +0xc8

github.com/couchbase/query/expression.(*Field).Evaluate(0xc42015ba90, 0x0, 0x0, 0x50a1c00, 0xc420fea000, 0x463504e, 0xc4201bc518, 0xc4201bc590, 0x4011692)

    /Users/xx/master/query/src/github.com/couchbase/query/expression/nav_field.go:52 +0x70

github.com/couchbase/query/expression.(*UnaryFunctionBase).UnaryEval(0xc42015bc20, 0x5097cc0, 0xc42015bc20, 0x0, 0x0, 0x50a1c00, 0xc420fea000, 0x0, 0x20, 0x28, ...)

    /Users/xx/master/query/src/github.com/couchbase/query/expression/function.go:332 +0x76

github.com/couchbase/query/expression.(*LikePrefix).Evaluate(0xc42015bc20, 0x0, 0x0, 0x50a1c00, 0xc420fea000, 0x8, 0x48dd480, 0xc4201bc5e0, 0x4011ee8)

    /Users/xx/master/query/src/github.com/couchbase/query/expression/func_like.go:56 +0x70

github.com/couchbase/query/execution.evalOne(0x50bbfc0, 0xc42015bc20, 0xc420fea000, 0x0, 0x0, 0x1, 0x0, 0x0, 0x0, 0xc420f75600)

    /Users/xx/master/query/src/github.com/couchbase/query/execution/scan_util.go:21 +0x1e8

github.com/couchbase/query/execution.evalSpan2(0xc4207262e0, 0x1, 0x1, 0xc420fea000, 0x406f840, 0xc4201bc748, 0xc4201bc740, 0xc420f6ba40, 0xc4201bc768, 0x406f84e)

    /Users/xx/master/query/src/github.com/couchbase/query/execution/scan_index2.go:191 +0x270

github.com/couchbase/query/execution.(*IndexScan2).scan(0xc420de0900, 0xc420fea000, 0xc421002120)

    /Users/xx/master/query/src/github.com/couchbase/query/execution/scan_index2.go:146 +0x9f

created by github.com/couchbase/query/execution.(*IndexScan2).RunOnce.func1

    /Users/xx/master/query/src/github.com/couchbase/query/execution/scan_index2.go:62 +0x1f2