OTP graph build fails when transfers.txt references parent station

[landonreed]

When building a graph for GTFS with transfers.txt, OTP fails if the file has an entry that references a parent station in either from_stop_id or to_stop_id

Exception thrown:

21:29:12.905 INFO (GTFSPatternHopFactory.java:594) Done finding interlining trips and creating the corresponding edges.
21:29:12.906 ERROR (OTPExceptionMapper.java:16) Unhandled exception
java.lang.NullPointerException: null
    at org.opentripplanner.routing.edgetype.factory.GTFSPatternHopFactory.loadTransfers(GTFSPatternHopFactory.java:1067) ~[otp-1.1.0-shaded.jar:1.1]
    at org.opentripplanner.routing.edgetype.factory.GTFSPatternHopFactory.run(GTFSPatternHopFactory.java:489) ~[otp-1.1.0-shaded.jar:1.1]
    at org.opentripplanner.graph_builder.module.GtfsModule.buildGraph(GtfsModule.java:122) ~[otp-1.1.0-shaded.jar:1.1]
    at org.opentripplanner.graph_builder.GraphBuilder.run(GraphBuilder.java:157) ~[otp-1.1.0-shaded.jar:1.1]
    at org.opentripplanner.api.resource.Routers.buildGraphOverWire(Routers.java:276) ~[otp-1.1.0-shaded.jar:1.1]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at ...```