Fix server crash due to invalid images

See https://bugs.freedesktop.org/show_bug.cgi?id=39383

Signed-off-by: Kirill Elagin <kirelagin@gmail.com>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>

fb/fbpict.c

@@ -163,7 +163,9 @@ create_bits_picture (PicturePtr pict,
 	(pixman_format_code_t)pict->format,
 	pixmap->drawable.width, pixmap->drawable.height,
 	(uint32_t *)bits, stride * sizeof (FbStride));
-    
+
+    if (!image)
+	return NULL;
     
 #ifdef FB_ACCESS_WRAPPER
 #if FB_SHIFT==5