audioflinger: Add Null pointer check while enabling effects for LPA

- MediaServer is crashing when disabling effects after mediaplayer
object is released.
- EffectChain pointer is de-referenced without NULL check, hence
leading to a mediaserver crash.
- Adding a NULL pointer check before using EffectChain object.

CRs-Fixed: 583305

Change-Id: If5ff469445246028046ba65e5ddf3300180318c2