MediaPlayer: use local variable to check member is null.
MediaPlayer: use local variable to check member is null.

Bug: 26961341
Change-Id: I343b18f4bfaba9b119a6721e1b39415474149d75

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base