wined3d: Print an ERR instead of crashing in context_check_fbo_status().

Some version of Mesa will claim the window system provided framebuffer is
incomplete when using indirect rendering, although the framebuffer objects
spec says this can never happen. Printing an ERR instead of crashing should
make it clearer that this isn't a wined3d problem.

dlls/wined3d/context.c

@@ -288,6 +288,12 @@ static void context_check_fbo_status(struct wined3d_context *context)
         unsigned int i;
         FIXME("FBO status %s (%#x)\n", debug_fbostatus(status), status);
 
+        if (!context->current_fbo)
+        {
+            ERR("FBO 0 is incomplete, driver bug?\n");
+            return;
+        }
+
         /* Dump the FBO attachments */
         for (i = 0; i < gl_info->max_buffers; ++i)
         {