Make removal of non-existent cache span a no-op

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=155413733

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -286,7 +286,9 @@ public final class SimpleCache implements Cache {
 
   private void removeSpan(CacheSpan span, boolean removeEmptyCachedContent) throws CacheException {
     CachedContent cachedContent = index.get(span.key);
-    Assertions.checkState(cachedContent.removeSpan(span));
+    if (cachedContent == null || !cachedContent.removeSpan(span)) {
+      return;
+    }
     totalSpace -= span.length;
     if (removeEmptyCachedContent && cachedContent.isEmpty()) {
       index.removeEmpty(cachedContent.key);