Dragon cannot be respawned in an old world

To reproduce:

1. Generate a world in a version before 1.9, and kill the dragon so that the exit portal is generated somewhere other than at (0,0).

2. Load the world in 1.10.1 or later.

3. Place the end crystals. See that the respawning process does not start.

It works fine in 1.10.

Further, as each crystal is placed, the following will be logged:

[08:53:13] [Server thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
	at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_91]
	at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_91]
	at h.a(SourceFile:46) [1.10.1.jar:?]
	at net.minecraft.server.MinecraftServer.D(SourceFile:598) [1.10.1.jar:?]
	at net.minecraft.server.MinecraftServer.C(SourceFile:554) [1.10.1.jar:?]
	at bzl.C(SourceFile:155) [1.10.1.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:458) [1.10.1.jar:?]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_91]
Caused by: java.lang.NullPointerException
	at atq.e(SourceFile:461) ~[1.10.1.jar:?]
	at adj.a(SourceFile:60) ~[1.10.1.jar:?]
	at adz.a(SourceFile:143) ~[1.10.1.jar:?]
	at lv.a(SourceFile:339) ~[1.10.1.jar:?]
	at me.a(SourceFile:627) ~[1.10.1.jar:?]
	at jj.a(SourceFile:55) ~[1.10.1.jar:?]
	at jj.a(SourceFile:11) ~[1.10.1.jar:?]
	at fl$1.run(SourceFile:13) ~[1.10.1.jar:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_91]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_91]
	at h.a(SourceFile:45) ~[1.10.1.jar:?]
	... 5 more

What seems to be happening (using MCP names) is that the fix for MC-103497 made DragonFightManager.respawnDragon() always try to use exitPortalLocation, even though findExitPortal() only sets this field when the detected portal is at (0,0). It should instead use the value returned by findExitPortal(), properly offset of course.