Instead of allocating a fixed amount of 128 bytes, which will overflow
and segfault, adjust the allocation to the length of the file name, and
if necessary trim the file name to make the prompt fit on the screen.

This fixes https://savannah.gnu.org/bugs/?47511

.
Reported-by: Aapo Rantalainen <aapo.rantalainen@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@justemail.net>