libfdt: overlay_rename_fragments: handle no fragments in base dtb

In case where base dtbo has no fragments, count_fragments returns
-FDT_ERR_NOTFOUND. In this case, there is no need to rename fragments in
the merged dtbo.

Change-Id: Id21c5a4c582b5b16059dae5c954c451a29a49240
Signed-off-by: Elliot Berman <eberman@codeaurora.org>