Handling destruction of empty Net objects

[ShaggO]

When constructing a network, the internal object initialization of Matlab actually creates a new network and replaces the already instantiated object, causing an empty network to be deleted. This PR checks if the caffe net pointer is defined before calling the net-deletion-function in caffe_.cpp.
Fixes #5346

[shelhamer]

Thanks for the fix!

[meixiaofeng]

This fix may still lead to error,I change the code to this, and everything gets all right:
if self.isvalid caffe_('delete_net', self.hNet_self); end

[mingzizongyouchongtu]

The similar issue that I have encountered as follow:
Error using caffe_
Usage: caffe_('delete_solver', hSolver)

Error in caffe.Solver/delete (line 40)
caffe_('delete_solver', self.hSolver_self);

The solution to this issue as the same to @meixiaofeng way. I change the code to as follow:
if self.isvalid
caffe_('delete_solver', self.hSolver_self);
end

Finally, thx to @meixiaofeng.