in copy_from_file(), fix potential segfault after uncutting one line of

in copy_from_file(), fix potential segfault after uncutting one line of
text with the mark on by properly preserving the beginning of the mark;
also, make sure the mark is always properly positioned after uncutting
one line of text with the mark on

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4141 35c25a1d-7b9e-4130-9fde-d3aeb78583b8