Alpha crash with shortest path query with password predicate

[campoy]

If you suspect this could be a bug, follow the template.

• What version of Dgraph are you using?

docker latest and play.dgraph.io

• Have you tried reproducing the issue with latest release?

Yes

• What is the hardware spec (RAM, OS)?

Various N/A

• Steps to reproduce the issue (command/config used to run Dgraph).

Send a query with a predicate containing at least one predicate of type password.
For instance, you can use the internal dgraph.password, but any other will also cause the issue.

{
  path as shortest(from: 0x1, to: 0x2) {
	  <dgraph.password>
  }
  path(func: uid(path)) {
  	name
	}
}

• Expected behaviour and actual result.

I expected to get a result, instead I get a panic:

panic: runtime error: index out of range

goroutine 2248 [running]:
github.com/dgraph-io/dgraph/query.(*SubGraph).expandOut(0xc0015d2000, 0x4f997e0, 0xc012fc8f30, 0xc0148591a0, 0xc012fc22a0, 0xc016d28a80)
	/Users/francesc/src/github.com/dgraph-io/dgraph/query/shortest.go:186 +0xfca
created by github.com/dgraph-io/dgraph/query.shortestPath
	/Users/francesc/src/github.com/dgraph-io/dgraph/query/shortest.go:455 +0x226