MT#59962 core/MixerBufferState: fix mem leak

Make sure mixed_channel is freed in the destructor.

Change-Id: I97c10cf38b6f30308b6a3a6b5eb7099867587bb3
Warned-by: Coverity

core/AmMultiPartyMixer.cpp

@@ -324,6 +324,7 @@ MixerBufferState::MixerBufferState(const MixerBufferState& other)
 
 MixerBufferState::~MixerBufferState()
 {
+  free_channels();
 }
 
 void MixerBufferState::add_channel(unsigned int channel_id)
@@ -369,5 +370,8 @@ void MixerBufferState::free_channels()
       delete it->second;
   }
 
-  delete mixed_channel;
+  if (mixed_channel) {
+    delete mixed_channel;
+    mixed_channel = NULL;
+  }
 }