Fix segfault for empty spare modules

src/storm-dft/storage/dft/DFT.cpp

@@ -754,6 +754,8 @@ namespace storm {
             // Check independence of spare modules
             // TODO: comparing one element of each spare module sufficient?
             for (auto module1 = mSpareModules.begin(); module1 != mSpareModules.end(); ++module1) {
+                if (!module1->second.empty()) {
+                    // Empty modules are allowed for the primary module of a spare gate
                     size_t firstElement = module1->second.front();
                     for (auto module2 = std::next(module1); module2 != mSpareModules.end(); ++module2) {
                         if (std::find(module2->second.begin(), module2->second.end(), firstElement) != module2->second.end()) {
@@ -765,6 +767,7 @@ namespace storm {
                         }
                     }
                 }
+            }
 
             if (validForAnalysis) {
                 // Check that each dependency is binary