fix debian prerm script (arangodb#4673)

ObiWahn · jsteemann · commit abde907eae8a · 2018-02-27T11:21:24.000+01:00
diff --git a/Installation/debian/prerm.in b/Installation/debian/prerm.in
@@ -7,6 +7,8 @@ if [ -x "/etc/init.d/arangodb3" ]; then
 fi
 
 if [ -d /run/systemd/system ] ; then
-        deb-systemd-invoke stop @CPACK_PACKAGE_NAME@.service >/dev/null
+        if systemctl list-unit-files | grep -q @CPACK_PACKAGE_NAME@.service ; then
+            deb-systemd-invoke stop @CPACK_PACKAGE_NAME@.service
+        fi
 fi
 # End automatically added section
