SpatializerHelper: fix the null pointer exception occurred when create spatializer

// the NullPointerException log
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: AudioService
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke interface method 'byte android.media.ISpatializer.getActualHeadTrackingMode()' on a null object reference
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.SpatializerHelper.createSpat(SpatializerHelper.java:965)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.SpatializerHelper.setSpatializerEnabledInt(SpatializerHelper.java:799)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.SpatializerHelper.reset(SpatializerHelper.java:337)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.AudioService.onAudioServerDied(AudioService.java:2134)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.AudioService$AudioHandler.handleMessage(AudioService.java:10295)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at android.os.Handler.dispatchMessage(Handler.java:106)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at android.os.Looper.loopOnce(Looper.java:224)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at android.os.Looper.loop(Looper.java:318)
11-15 07:34:17.191  1000  2107  4695 E AndroidRuntime:  at com.android.server.audio.AudioService$AudioSystemThread.run(AudioService.java:10067)

Change-Id: I0d43b6f4ace86a9db8bb1359dbaf5c28241b8cb4
Signed-off-by: wumin3 <wumin3@xiaomi.com>