common: avoid calling umount without any args

For example, when calling arch-chroot on a dir without a /proc:

  # arch-chroot badroot
  mount: badroot/proc: mount point does not exist.
  ==> ERROR: failed to setup chroot badroot
  umount: bad usage
  Try 'umount --help' for more information.

common

@@ -91,7 +91,9 @@ chroot_setup() {
 }
 
 chroot_teardown() {
-  umount "${CHROOT_ACTIVE_MOUNTS[@]}"
+  if (( ${#CHROOT_ACTIVE_MOUNTS[@]} )); then
+    umount "${CHROOT_ACTIVE_MOUNTS[@]}"
+  fi
   unset CHROOT_ACTIVE_MOUNTS
 }