Preventing a floating-point exception when the available length

Preventing a floating-point exception when the available length
for an answer becomes zero.  The answer will instead wrap to the
next line, which does not look nice but is better than crashing.
Patch by Mahyar Abbaspour.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@5235 35c25a1d-7b9e-4130-9fde-d3aeb78583b8