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>
(cherry picked from commit 2906ee5e4a)

glamor/glamor_program.c

@@ -383,6 +383,8 @@ fail:
         glDeleteProgram(prog->prog);
         prog->prog = 0;
     }
+    free(vs_prog_string);
+    free(fs_prog_string);
     free(version_string);
     free(fs_vars);
     free(vs_vars);