Fixed bug 1313 - Segfault on SDL_CreateWindow when gl lib cannot be loaded

Don't crash if the gl_data isn't valid, just return NULL.

src/video/x11/SDL_x11opengl.c

@@ -187,8 +187,10 @@ X11_GL_UnloadLibrary(_THIS)
 #endif
 
     /* Free OpenGL memory */
+    if (_this->gl_data) {
         SDL_free(_this->gl_data);
         _this->gl_data = NULL;
+    }
 }
 
 static SDL_bool
@@ -396,6 +398,11 @@ X11_GL_GetVisual(_THIS, Display * display, int screen)
     const int i = X11_GL_GetAttributes(_this,display,screen,attribs,max_attrs);
     SDL_assert(i <= max_attrs);
 
+    if (!_this->gl_data) {
+        /* The OpenGL library wasn't loaded, SDL_GetError() should have info */
+        return NULL;
+    }
+
     vinfo = _this->gl_data->glXChooseVisual(display, screen, attribs);
     if (!vinfo) {
         SDL_SetError("Couldn't find matching GLX visual");