painting: do not bluntly ignore zero-length start matches -- handle them

The segmentation fault that this causes when both start and end match are
zero-length will be tackled later (https://savannah.gnu.org/bugs/?50056).

This fixes https://savannah.gnu.org/bugs/?50078.
Inspired-by: Elia Geretto <elia.f.geretto@gmail.com>
1 file changed