read_line: return empty line if ifstream fails

src/file_utils.cpp

@@ -14,6 +14,9 @@ std::string read_line(const std::string& filename)
 {
     std::string line;
     std::ifstream file(filename);
+    if (file.fail()){
+        return line;
+    }
     std::getline(file, line);
     return line;
 }