LU-12703 utils: reset rootpath in llapi_search_rootpath()

as get_root_path() can use it as a source and fail if
passed pathname contains garbage (on stack);

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I9f628353c872afc82a582b0a6ca960cd0e8cffcb
Reviewed-on: https://review.whamcloud.com/36335
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>