Handle NULL fb_ptr in pixmap_get_fb

This can happen when HW acceleration is disabled.

Fixes xorg/driver/xf86-video-ati#188
(ported from radeon commit 4d84cf438e7f1bebf0053035ef0292e9fed257d1)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
13 jobs for handle-null-fb_ptr in 2 minutes and 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1973596
debian-testing

00:00:40

failed #1973157
debian-testing

00:03:09

 
  Build
passed #1973160
xserver-1.13

00:00:35

passed #1973161
xserver-1.14

00:00:33

passed #1973162
xserver-1.15

00:00:31

passed #1973163
xserver-1.16

00:00:31

passed #1973164
xserver-1.17

00:00:31

passed #1973165
xserver-1.18

00:00:43

passed #1973166
xserver-1.18-clang

00:00:39

passed #1973167
xserver-1.19

00:00:34

passed #1973168
xserver-1.19-clang

00:00:40

passed #1973158
xserver-1.20

00:00:56

passed #1973159
xserver-1.20-clang

00:00:35