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>
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