Initialize shared pointers before use

Should address runtime errors reported in #29

pcl_ros/src/pcl_ros/filters/voxel_grid.cpp

@@ -58,7 +58,7 @@ pcl_ros::VoxelGrid::filter (const PointCloud2::ConstPtr &input,
                             PointCloud2 &output)
 {
   boost::mutex::scoped_lock lock (mutex_);
-  pcl::PCLPointCloud2::Ptr pcl_input;
+  pcl::PCLPointCloud2::Ptr pcl_input(new pcl::PCLPointCloud2);
   pcl_conversions::toPCL (*(input), *(pcl_input));
   impl_.setInputCloud (pcl_input);
   impl_.setIndices (indices);