prevent crash if no string to draw

Fltk/drawContextFltkCairo.cpp

@@ -56,6 +56,7 @@ class drawContextFltkCairo::queueString {
 
   void flush()
   {
+    if(_elements.empty()) return;
     cairo_surface_t *surface = cairo_image_surface_create(CAIRO_FORMAT_A8, _totalWidth, _maxHeight);
     cairo_t *cr = cairo_create(surface);
     int pos = 0;

Fltk/drawContextFltkStringTexture.cpp

@@ -45,7 +45,6 @@ class drawContextFltkStringTexture::queueString {
   void flush()
   {
     if(_elements.empty()) return;
-
     //1000 should be _totalWidth but it does not work
     int w = 1000, h = _maxHeight;
     Fl_Offscreen offscreen = fl_create_offscreen(w, h);