Bazel-Install.bat.tpl: check %BAZEL_VERSION%

Bazel-Install-Trigger #792 and
Bazel-Install #16393 failed because
%BAZEL_VERSION% was empty. (Why, I don't know.)

Change-Id: I0d80c72726e436cf4f0e6fcc8e4b666327a8e14c
diff --git a/jenkins/jobs/Bazel-Install.bat.tpl b/jenkins/jobs/Bazel-Install.bat.tpl
index e50eddb..6835a37 100644
--- a/jenkins/jobs/Bazel-Install.bat.tpl
+++ b/jenkins/jobs/Bazel-Install.bat.tpl
@@ -24,7 +24,11 @@
 for /F %%i in ('powershell -file get_latest_bazel_version.ps1') do set BAZEL_VERSION=%%i
 del get_latest_bazel_version.ps1
 
-echo %BAZEL_VERSION%
+echo BAZEL_VERSION=(%BAZEL_VERSION%)
+if "%BAZEL_VERSION%" == "" (
+  echo ERROR: Could not parse BAZEL_VERSION from https://github.com/bazelbuild/bazel/releases/latest
+  exit 1
+)
 
 :: Download the latest bazel release
 set folder=c:\bazel_ci\installs\%BAZEL_VERSION%