nfs_rename_continue_1_internal: fix use-after-free

Spotted by clang analyzer.
This introduces another allocation to create a copy of the target path
of a rename in case it needs to be reported via rpc_set_error - it might
be a better idea to avoid the allocation and have a slightly less informative
error message?

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>