Fix mem free bug and null pointer error

When exiting libfreenect2::CpuDepthPacketProcessor::process() is
called but listener_ pointer is NULL. Adding checking to listener_.

First time deleting not alloced mem pointer buffer_ will fail.
When creating Mat buffer_ set it to NULL.

Author: yuanmingze

examples/protonect/src/cpu_depth_packet_processor.cpp

@@ -107,7 +107,7 @@ struct Mat
 
 public:
   /** Default constructor. */
-  Mat()
+  Mat():buffer_(0), buffer_end_(0)
   {
   }
 
@@ -1091,17 +1091,20 @@ void CpuDepthPacketProcessor::process(const DepthPacket &packet)
       }
   }
 
-  if(listener_->onNewFrame(Frame::Ir, impl_->ir_frame))
-  {
-    impl_->newIrFrame();
-  }
+  impl_->stopTiming(LOG_INFO);
 
-  if(listener_->onNewFrame(Frame::Depth, impl_->depth_frame))
-  {
-    impl_->newDepthFrame();
+  if (listener_ != 0 ){
+    if(listener_->onNewFrame(Frame::Ir, impl_->ir_frame))
+    {
+      impl_->newIrFrame();
+    }
+
+    if(listener_->onNewFrame(Frame::Depth, impl_->depth_frame))
+    {
+      impl_->newDepthFrame();
+    }
   }
 
-  impl_->stopTiming(LOG_INFO);
 }
 
 } /* namespace libfreenect2 */