Correctly manage the case of string=nil if wreng_string_t is string_view

src/def_configuration.cpp

@@ -20,12 +20,28 @@
 #include <algorithm>
 
 namespace wren {
+	namespace {
+		template <typename T>
+		bool empty(T* message) {
+			return not (message and *message);
+		}
+	} //unnamed namespace
+
 	void DefConfiguration::write_fn (VM*, wren_string_t text) {
 		std::cout << text;
 	}
 
 	void DefConfiguration::error_fn (VM*, ErrorType type, wren_string_t module, int line, wren_string_t message) {
-		std::cerr << "Wren error: " << message << " in " << module << ':' << line << '\n';
+		using std::empty;
+
+		std::cerr << "Wren error: " << message;
+		if (!empty(module)) {
+			std::cerr << " in " << module;
+			if (line > -1) {
+				std::cerr << ':' << line;
+			}
+		}
+		std::cerr << '\n';
 	}
 
 	void* DefConfiguration::reallocate_fn (void* ptr, std::size_t size) {

src/vm.cpp

@@ -47,7 +47,8 @@ namespace wren {
 			auto cb = static_cast<detail::Callbacks*>(wrenGetUserData(wvm));
 			assert(cb);
 			assert(cb->error_fn and cb->config_obj and cb->owner);
-			cb->error_fn(*cb->config_obj, cb->owner, to_error_type(type), module, line, message);
+			const char* const sane_module = (module ? module : "");
+			cb->error_fn(*cb->config_obj, cb->owner, to_error_type(type), sane_module, line, message);
 		}
 
 		const char* resolve_module_fn (WrenVM* wvm, const char* importer, const char* name) {