Fix build on gcc 4.4

test/util.h

@@ -35,7 +35,7 @@ std::string get_system_error(int error_code);
 extern const char *const FILE_CONTENT;
 
 // Opens a buffered file for reading.
-fmt::buffered_file open_buffered_file(FILE **fp = nullptr);
+fmt::buffered_file open_buffered_file(FILE **fp = FMT_NULL);
 
 inline FILE *safe_fopen(const char *filename, const char *mode) {
 #if defined(_WIN32) && !defined(__MINGW32__)