Fixes issue #5979 - NULL pointer crash in ModelObject::split()

ModelObject::split() expects a non-NULL new_objects vector where it adds pointers to the new models resulting from the split.
But in the CLI case the caller does not care about this and passes NULL which causes a crash. To fix the crash we could pass
a dummy vector but it turns out that we actually have a use for the results because we should assign a unique name to each
new model the same way as the GUI does. These names show up as comments in the gcode so this change makes the gcode produced
by the GUI and the CLI more similar and diffable.

@lukasmatena has amended the original commit by @combolek (pull request #5991) in order to avoid code duplication

src/PrusaSlicer.cpp

@@ -402,7 +402,8 @@ int CLI::run(int argc, char **argv)
             for (Model &model : m_models) {
                 size_t num_objects = model.objects.size();
                 for (size_t i = 0; i < num_objects; ++ i) {
-                    model.objects.front()->split(nullptr);
+                    ModelObjectPtrs new_objects;
+                    model.objects.front()->split(&new_objects);
                     model.delete_object(size_t(0));
                 }
             }

src/libslic3r/Model.cpp

@@ -1305,6 +1305,7 @@ void ModelObject::split(ModelObjectPtrs* new_objects)
 
     ModelVolume* volume = this->volumes.front();
     TriangleMeshPtrs meshptrs = volume->mesh().split();
+    size_t counter = 1;
     for (TriangleMesh *mesh : meshptrs) {
 
         // FIXME: crashes if not satisfied
@@ -1314,7 +1315,8 @@ void ModelObject::split(ModelObjectPtrs* new_objects)
 
         // XXX: this seems to be the only real usage of m_model, maybe refactor this so that it's not needed?
         ModelObject* new_object = m_model->add_object();    
-        new_object->name   = this->name;
+        new_object->name   = this->name + (meshptrs.size() > 1 ? "_" + std::to_string(counter++) : "");
+
         // Don't copy the config's ID.
 		new_object->config.assign_config(this->config);
 		assert(new_object->config.id().valid());

src/slic3r/GUI/Plater.cpp

@@ -2892,10 +2892,6 @@ void Plater::priv::split_object()
     {
         Plater::TakeSnapshot snapshot(q, _L("Split to Objects"));
 
-        unsigned int counter = 1;
-        for (ModelObject* m : new_objects)
-            m->name = current_model_object->name + "_" + std::to_string(counter++);
-
         remove(obj_idx);
 
         // load all model objects at once, otherwise the plate would be rearranged after each one