Check that /system exists in chroot before cleaning it up.

Test: adb shell rm -rf "$ART_TEST_CHROOT/system" && art/tools/cleanup-buildbot-device.sh
Bug: 34729697
Change-Id: Icd5a1e0dd35480704c468a4e738f88a86ab85f8f
diff --git a/tools/cleanup-buildbot-device.sh b/tools/cleanup-buildbot-device.sh
index 53072ae..8c28828 100755
--- a/tools/cleanup-buildbot-device.sh
+++ b/tools/cleanup-buildbot-device.sh
@@ -44,10 +44,11 @@
   # TODO: Also consider adding a "tear down device" step on the ART
   # Buildbot (at the very end of a build) undoing (some of) the work
   # done in the "device setup" step.
-  adb shell find "$ART_TEST_CHROOT/system" \
-    ! -path "$ART_TEST_CHROOT/system/etc/selinux/plat_property_contexts" \
-    ! -type d \
-    -exec rm -f \{\} +
+  adb shell test -f "$ART_TEST_CHROOT/system" \
+    "&&" find "$ART_TEST_CHROOT/system" \
+      ! -path "$ART_TEST_CHROOT/system/etc/selinux/plat_property_contexts" \
+      ! -type d \
+      -exec rm -f \{\} +
 
   echo -e "${green}Clean up some subdirs in /data in chroot${nc}"
   adb shell rm -rf \