#59 Fix segfault triggered by fbcon driver in graphics.c

[Tobi Vollebregt]

I'm trying to use Allegro 4.4 on an embedded platform (ea3250 development board), using the Linux console and fbcon subsystems.

After applying the hack from #3556659, I got a segmentation fault in graphics.c line 660. This line (and the two below it) access the screen pointer using bmp_read_line, bmp_write_line and bmp_unwrite_line. When setting a GFX_FBCON graphics mode, apparently the fbcon driver calls _set_gfx_mode when gfx_driver != NULL and screen == NULL at some point, thus causing a segmentation fault.

The attached patch (applies to 4.4 branch) fixes this by checking if screen != NULL before calling bmp_read_line et. al..

[Tobi Vollebregt]

Fix segmentation fault if screen == 0