Fix NPE if source is released having never been prepared.

library/src/main/java/com/google/android/exoplayer/FrameworkSampleSource.java

@@ -234,7 +234,7 @@ public final class FrameworkSampleSource implements SampleSource {
   @Override
   public void release() {
     Assertions.checkState(remainingReleaseCount > 0);
-    if (--remainingReleaseCount == 0) {
+    if (--remainingReleaseCount == 0 && extractor != null) {
       extractor.release();
       extractor = null;
     }

library/src/main/java/com/google/android/exoplayer/extractor/ExtractorSampleSource.java

@@ -313,7 +313,7 @@ public class ExtractorSampleSource implements SampleSource, ExtractorOutput, Loa
   @Override
   public void release() {
     Assertions.checkState(remainingReleaseCount > 0);
-    if (--remainingReleaseCount == 0) {
+    if (--remainingReleaseCount == 0 && loader != null) {
       loader.release();
       loader = null;
     }