Fix compilation warnings (#327)

Signed-off-by: mini-1235 <mauricepurnawan@gmail.com>

Author: mini-1235

spatio_temporal_voxel_layer/src/spatio_temporal_voxel_grid.cpp

@@ -124,7 +124,7 @@ void SpatioTemporalVoxelGrid::ClearFrustums(
   std::vector<observation::MeasurementReading>::const_iterator it =
     clearing_readings.begin();
   for (; it != clearing_readings.end(); ++it) {
-    geometry::Frustum * frustum;
+    geometry::Frustum * frustum = nullptr;
     if (it->_model_type == DEPTH_CAMERA) {
       frustum = new geometry::DepthCameraFrustum(
         it->_vertical_fov_in_rad,