handle empty dirs when migrating distdirs

If there are no files in the dir that match the glob, or there are
broken symlinks, don't try migrating them over.

BUG=None
TEST=CQ passes

Change-Id: I350680594b35d2df6065113afe0ce3447ee13e1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2290711
Reviewed-by: Kalvin Lee <kdlee@chromium.org>
Tested-by: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/chroot_version_hooks.d/178_merge_distdir b/chroot_version_hooks.d/178_merge_distdir
index f0598a2..ff79034 100644
--- a/chroot_version_hooks.d/178_merge_distdir
+++ b/chroot_version_hooks.d/178_merge_distdir
@@ -36,7 +36,7 @@
   # Merge the files.
   rm -f "${srcdir}"/*._checksum_failure_.*
   for f in "${srcdir}"/*; do
-    if [[ ! -e "${f##*/}" ]]; then
+    if [[ -e "${f}" && ! -e "${f##*/}" ]]; then
       mv "${f}" ./ || exit 1
     fi
   done