Fix NPE in MediaControllerImplLegacy#connectToSession()

[h6ah4i]

This pull request fixes a NullPointerException occurs inside of the MediaControllerImplLegacy#connectToSession().

2022-03-27 00:28:23.683 31182-31211/? E/AndroidRuntime: FATAL EXCEPTION: MCwMSCTest
    Process: androidx.media3.test.session, PID: 31182
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v4.media.session.MediaControllerCompat.isSessionReady()' on a null object reference
        at androidx.media3.session.MediaControllerImplLegacy.lambda$connectToSession$2$androidx-media3-session-MediaControllerImplLegacy(MediaControllerImplLegacy.java:1274)
        at androidx.media3.session.MediaControllerImplLegacy$$ExternalSyntheticLambda14.run(Unknown Source:2)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loopOnce(Looper.java:201)
        at android.os.Looper.loop(Looper.java:288)
        at android.os.HandlerThread.run(HandlerThread.java:67)

Where the NPE occurs

media/libraries/session/src/main/java/androidx/media3/session/MediaControllerImplLegacy.java

Line 1274 in 72a4fb0

if (!controllerCompat.isSessionReady()) {

Scenario

1. Create a MediaController instance
   • 👉 MediaControllerImplLegacy posts a callback which is executed on the main thread
2. Call MediaController#release() immediately on the main thread
   • 👉 MediaControllerImplLegacy sets controllerCompat = null
3. The callback registered at Step 2 is invoked
   • 👉 controllerCompat.isSessionReady() is called 💥