.gitlab-ci: verify that Get-Content worked

This will hopefully prevent future breakages similar to what the
previous commit fixed.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8052>

.gitlab-ci/windows/piglit_run.ps1

@@ -7,8 +7,13 @@ cmd.exe /C "C:\BuildTools\Common7\Tools\VsDevCmd.bat -host_arch=amd64 -arch=amd6
 
 py -3 C:\Piglit\bin\piglit.py summary console .\results | Select -SkipLast 1 | Select-String -NotMatch -Pattern ': pass' | Set-Content -Path .\result.txt
 
-$diff = Compare-Object -ReferenceObject $(Get-Content ".\_install\$env:PIGLIT_PROFILE.txt") `
-                       -DifferenceObject $(Get-Content .\result.txt)
+$reference = Get-Content ".\_install\$env:PIGLIT_PROFILE.txt"
+$result = Get-Content .\result.txt
+if (-Not ($reference -And $result)) {
+  Exit 1
+}
+
+$diff = Compare-Object -ReferenceObject $reference -DifferenceObject $result
 if (-Not $diff) {
   Exit 0
 }