LU-18896 utils: don't miss null-terminator in append_param

LU-18896 utils: don't miss null-terminator in append_param

The code to move tail can skip null-terminator if there is
no tail and parameter is the last one.

Fixes: f9d346f0c1 ("LU-18587 utils: allow large NID lists in mount tools")
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I088f7a0eda80a9df1d55b19db28793aa488cc95e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58685
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>