glamor: Fix leak in glamor_build_program()

Fix the possible leak of `vs_prog_string` and `fs_prog_string` in case
of failure, as reported by covscan.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
5 jobs for covscan-fixes in 5 minutes and 29 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #13681943

00:00:23

 
  Build And Test
passed autotools #13681944

00:05:06

passed meson #13681945

00:02:06

passed meson-noglamor #13681946

00:01:27

passed mingw-cross-build #13681947

00:00:41