in resampling, sassert that there are slices

Resample/Processed/ReSample.cpp

@@ -305,6 +305,7 @@ OwningVector<const ReLayout> collectLayouts(const MultiLayer& sample, const Slic
 {
     OwningVector<const ReLayout> result;
 
+    ASSERT(!slices.empty());
     double z_ref = -slices.front().low();
 
     for (size_t i = 0; i < sample.numberOfLayers(); ++i) {