Handle some Exceptions caused eg. by missing network during shutdown

[funtax]

If you close the player for example during missing network or while a reconnect is in progress, some issues might occur and cause uncaught Exceptions.
This PR fixes the most occurring issues.

This might especially be required if you use librespot-java on a mobile client like Android.

[devgianlu]

I should have a look at fixing this at the source instead of catching the exceptions afterwards.

[funtax]

This would be of course the best solution. Might be a little bit complicated to reproduce. It's a mix between "disconnect network" and closing the player+session. An often happening situation on mobile devices and will likely also affect the new Android-project of librespot-java.

[devgianlu]

I remember having dealt with that before, having a closer look now.

[devgianlu]

@funtax Do you have stacktraces for those NullPointerExceptions? I cannot keep them

[funtax]

They might be useless because they happen at another position now and are extracted from "Android Vitals".
I think it was with player-1.5.5, then I internally updated my custom version of librespot-java.
But maybe you can reproduce the situation:

java.lang.NullPointerException:
at xyz.gianlu.librespot.core.Session.reconnect (Session.java:671)
at xyz.gianlu.librespot.core.Session.access$1100 (Session.java:60)
at xyz.gianlu.librespot.core.Session$Receiver.run (Session.java:1295)
at java.lang.Thread.run (Thread.java:929)

java.lang.NullPointerException:
at xyz.gianlu.librespot.core.Session.connect (Session.java:299)
at xyz.gianlu.librespot.core.Session.reconnect (Session.java:669)
at xyz.gianlu.librespot.core.Session.access$1100 (Session.java:60)
at xyz.gianlu.librespot.core.Session$Receiver.run (Session.java:1295)
at java.lang.Thread.run (Thread.java:929)

And the one within sendUnchecked() might be triggered by "conn" set to "null" during Session.close() and thus might happen at several positions during closing the session.

[devgianlu]

I should probably refactor that part of the code at some point, but for now this should do.