aborts with 'empty' images

```
convert -size 80x60 xc:none /tmp/test.jpg
```

```
./SaliencyDetector /tmp/test.jpg
SaliencyDetector: ../src/SymmetricSurroundSaliency.h:129: void SymmetricSurroundSaliency::Normalize(std::vector<double>&, const int&, const int&): Assertion `range > 0' failed.
Aborted (core dumped)
```

could this be handled more gracefully? 
