glamor: Fix a compiler warning since the recent OOM fixes.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 3b26b90c)
3 jobs for server-1.20-branch in 4 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1588691
debian-testing

00:00:26

 
  Build And Test
passed #1588692
autotools-build-and-test

00:04:22

passed #1588693
meson-build-and-test

00:01:53