Recorder: Avoid NPE when screen recording is about to finish

 * The following crash can be reproduced in some devices by
   starting a screen recording and then stopping it from the
   sticky notification:

    E AndroidRuntime: FATAL EXCEPTION: Encoder
    E AndroidRuntime: Process: org.lineageos.recorder, PID: 3187
    E AndroidRuntime: java.lang.NullPointerException: println needs a message
    E AndroidRuntime:        at android.util.Log.println_native(Native Method)
    E AndroidRuntime:        at android.util.Log.e(Log.java:240)
    E AndroidRuntime:        at org.lineageos.recorder.screen.EncoderDevice$EncoderRunnable.run(EncoderDevice.java:320)
    E AndroidRuntime:        at java.lang.Thread.run(Thread.java:764)
    I ActivityManager: Showing crash dialog for package org.lineageos.recorder u0

 * This also matches other try...catch blocks used in other parts
   of the code.

Change-Id: I8cd1d2f0be250603b791dd7be7db85f62b3ac26e