Check that cache dir exist

Not checking it would force ExoPlayer to use the
global tmp dir which would expose it to
external file replacement attacks.

This is a theoretical vulnerability as this code is
only use in tests and cache dir always exist in the
AOSP android implementation.

PiperOrigin-RevId: 353013929

library/common/src/main/java/com/google/android/exoplayer2/util/Util.java

@@ -2084,7 +2084,7 @@ public final class Util {
 
   /** Creates a new empty file in the directory returned by {@link Context#getCacheDir()}. */
   public static File createTempFile(Context context, String prefix) throws IOException {
-    return File.createTempFile(prefix, null, context.getCacheDir());
+    return File.createTempFile(prefix, null, checkNotNull(context.getCacheDir()));
   }
 
   /**