Producing the correct return value for when a mouse event reinserted something

Producing the correct return value for when a mouse event reinserted something
into the keyboard buffer.  And not continuing but getting out when there is a
spurious  mouse event.  Continuing would result in the cutbuffer being cleared
upon the next cut.  These two changes together fix Savannah bug #42326.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4956 35c25a1d-7b9e-4130-9fde-d3aeb78583b8