Avoid warning:

../../../../ext/fox16_c/frames_wrap.cpp: In function ‘VALUE FXGradientBar_gradient(FX::FXGradientBar*, FX::FXint)’:
../../../../ext/fox16_c/frames_wrap.cpp:2465:37: warning: zero-length gnu_printf format string [-Wformat-zero-length]
           rb_raise(rb_eNoMemError,"");
                                     ^