Bug in insert mode completion when completeopts has 'menu,longest'

[justin constantino]

Sometimes the completed text gets erased after hitting enter.

To reproduce:

1. vim -u NONE +'set nocp cot=menu,longest'
2. ifoobar;<CR>fo<C-P>;
3. the two lines should be the same now.  press <CR> and the second
line then becomes "fo".