Add error checking to C style io

src/storm/storage/dd/sylvan/InternalSylvanBdd.cpp

@@ -234,15 +234,25 @@ namespace storm {
 
         void InternalBdd<DdType::Sylvan>::exportToDot(std::string const& filename, std::vector<std::string> const&, bool) const {
             FILE* filePointer = fopen(filename.c_str() , "a+");
+            // fopen returns a nullptr on failure
+            if (filePointer == nullptr) {
+                STORM_LOG_ERROR("Failure to open file: " << filename);
+            } else {
                     this->sylvanBdd.PrintDot(filePointer);
                     fclose(filePointer);
             }
+        }
 
         void InternalBdd<DdType::Sylvan>::exportToText(std::string const& filename) const {
             FILE* filePointer = fopen(filename.c_str() , "a+");
+            // fopen returns a nullptr on failure
+            if (filePointer == nullptr) {
+                STORM_LOG_ERROR("Failure to open file: " << filename);
+            } else {
                 this->sylvanBdd.PrintText(filePointer);
                 fclose(filePointer);
             }
+        }
 
         sylvan::Bdd& InternalBdd<DdType::Sylvan>::getSylvanBdd() {
             return sylvanBdd;