AudioMixer: fix crash in destructor

Commit deeb1282 made that track->resampler is only set to NULL when
getTrackName() is called. It remains uninitialized for all unused tracks
causing a crash when deleted by Audiomixer destructor.

Issue 6309441.

Change-Id: I49451387a6afe81299b8368ef206524df32f74c4
diff --git a/services/audioflinger/AudioMixer.cpp b/services/audioflinger/AudioMixer.cpp
index 399d987..05fa3b9 100644
--- a/services/audioflinger/AudioMixer.cpp
+++ b/services/audioflinger/AudioMixer.cpp
@@ -119,6 +119,7 @@
     for (unsigned i=0 ; i < MAX_NUM_TRACKS ; i++) {
         // FIXME redundant per track
         t->localTimeFreq = lc.getLocalFreq();
+        t->resampler = NULL;
         t++;
     }