In VoxelGrid, Assertion `px != 0' failed. 

Hi, I found one issue with the voxel grid class for hydro.

I'm running the voxel grid in nodelet manager as follows:
```
<node pkg="nodelet" type="nodelet" name="voxel_grid" args="load pcl/VoxelGrid sensor_manager" output="screen">
    <remap from="~input" to="/camera/depth/points" />
    <rosparam>
      filter_field_name: z
      filter_limit_min: 0.3
      filter_limit_max: 3
      filter_limit_negative: False
      leaf_size: 0.03
    </rosparam>
  </node>
```
when I tried to echo the output: /voxel_grid/output, the error comes as:
```
nodelet: /usr/include/boost/smart_ptr/shared_ptr.hpp:418: boost::shared_ptr<T>::reference boost::shared_ptr<T>::operator*() const [with T = pcl::PCLPointCloud2; boost::shared_ptr<T>::reference = pcl::PCLPointCloud2&]: Assertion `px != 0' failed.
```

I think it's from the voxel_grip.cpp:
```
pcl_conversions::toPCL (*(input), *(pcl_input));
```

I then modified the declaration of pcl_input to:
```
pcl::PCLPointCloud2::Ptr pcl_input (new pcl::PCLPointCloud2);
```
It works after this change. 

But.... the SACSegmentation nodelet is not working correctly. And I could not solve that problem related to memory leak...
```
*** glibc detected *** /opt/ros/hydro/lib/nodelet/nodelet: free(): invalid pointer: 0xb1f1abc0 ***
```

Can anyone please look into this issue?
Thanks!