Check referenceCount > 0 in DefaultDrmSession#release()

Also flip DefaultDrmSessionManager#prepare()/release() into guard
clauses.

Suggestions from review comments on:
https://github.com/google/ExoPlayer/commit/316f8a88cdaeff2b1b5d219097b2739ef607b2c4

PiperOrigin-RevId: 320572462

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java

@@ -15,6 +15,8 @@
  */
 package com.google.android.exoplayer2.drm;
 
+import static com.google.android.exoplayer2.util.Assertions.checkState;
+
 import android.annotation.SuppressLint;
 import android.media.NotProvisionedException;
 import android.os.Handler;
@@ -272,12 +274,12 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
   @Override
   public void acquire(@Nullable DrmSessionEventListener.EventDispatcher eventDispatcher) {
-    Assertions.checkState(referenceCount >= 0);
+    checkState(referenceCount >= 0);
     if (eventDispatcher != null) {
       eventDispatchers.add(eventDispatcher);
     }
     if (++referenceCount == 1) {
-      Assertions.checkState(state == STATE_OPENING);
+      checkState(state == STATE_OPENING);
       requestHandlerThread = new HandlerThread("ExoPlayer:DrmRequestHandler");
       requestHandlerThread.start();
       requestHandler = new RequestHandler(requestHandlerThread.getLooper());
@@ -295,6 +297,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
   @Override
   public void release(@Nullable DrmSessionEventListener.EventDispatcher eventDispatcher) {
+    checkState(referenceCount > 0);
     if (--referenceCount == 0) {
       // Assigning null to various non-null variables for clean-up.
       state = STATE_RELEASED;

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java

@@ -440,26 +440,28 @@ public class DefaultDrmSessionManager implements DrmSessionManager {
 
   @Override
   public final void prepare() {
-    if (prepareCallsCount++ == 0) {
-      Assertions.checkState(exoMediaDrm == null);
-      exoMediaDrm = exoMediaDrmProvider.acquireExoMediaDrm(uuid);
-      exoMediaDrm.setOnEventListener(new MediaDrmEventListener());
+    if (prepareCallsCount++ != 0) {
+      return;
     }
+    Assertions.checkState(exoMediaDrm == null);
+    exoMediaDrm = exoMediaDrmProvider.acquireExoMediaDrm(uuid);
+    exoMediaDrm.setOnEventListener(new MediaDrmEventListener());
   }
 
   @Override
   public final void release() {
-    if (--prepareCallsCount == 0) {
-      // Make a local copy, because sessions are removed from this.sessions during release (via
-      // callback).
-      List<DefaultDrmSession> sessions = new ArrayList<>(this.sessions);
-      for (int i = 0; i < sessions.size(); i++) {
-        // Release all the keepalive acquisitions.
-        sessions.get(i).release(/* eventDispatcher= */ null);
-      }
-      Assertions.checkNotNull(exoMediaDrm).release();
-      exoMediaDrm = null;
-    }
+    if (--prepareCallsCount != 0) {
+      return;
+    }
+    // Make a local copy, because sessions are removed from this.sessions during release (via
+    // callback).
+    List<DefaultDrmSession> sessions = new ArrayList<>(this.sessions);
+    for (int i = 0; i < sessions.size(); i++) {
+      // Release all the keepalive acquisitions.
+      sessions.get(i).release(/* eventDispatcher= */ null);
+    }
+    Assertions.checkNotNull(exoMediaDrm).release();
+    exoMediaDrm = null;
   }
 
   @Override