glamor: Check for NULL pixmap in glamor_get_pixmap_texture()

glamor_create_pixmap() would return a NullPixmap if the given size is
larger than the maximum size of a pixmap.

But glamor_get_pixmap_texture() won't check if the given pixmap is
non-null, leading to a segfault if glamor_create_pixmap() failed.

This can be reproduced by passing Xephyr a very large screen width,
e.g.:

 $ Xephyr -glamor -screen 32768x1024 :10

 (EE)
 (EE) Backtrace:
 (EE) 0: Xephyr (OsSigHandler+0x29)
 (EE) 1: /lib64/libpthread.so.0 (__restore_rt+0x0)
 (EE) 2: Xephyr (glamor_get_pixmap_texture+0x30)
 (EE) 3: Xephyr (ephyr_glamor_create_screen_resources+0xc6)
 (EE) 4: Xephyr (ephyrCreateResources+0x98)
 (EE) 5: Xephyr (dix_main+0x275)
 (EE) 6: /lib64/libc.so.6 (__libc_start_main+0xf1)
 (EE) 7: Xephyr (_start+0x2a)
 (EE) 8: ? (?+0x2a) [0x2a]
 (EE)
 (EE) Segmentation fault at address 0x0
 (EE)
 Fatal server error:
 (EE) Caught signal 11 (Segmentation fault). Server aborting
 (EE)
 Aborted (core dumped)

Bugzilla: https://bugzilla.redhat.com/1431633


Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit f40ff18c)