in real_dir_from_tilde(), fix long-standing problem where directory

in real_dir_from_tilde(), fix long-standing problem where directory
names that began with "~", but that weren't users' home directories,
could be erroneously treated as users' home directories (e.g. "~d/"
would be treated as "~daemon/")

git-svn-id: svn://svn.savannah.gnu.org/nano/branches/nano_2_0_branch/nano@4074 35c25a1d-7b9e-4130-9fde-d3aeb78583b8