model: Let get_n_columns() return a sane value

model: Let get_n_columns() return a sane value

If you call get_n_columns() during the instance initialization phase but
before set_name()/set_types() have been called, you'll get a (guint) -1.
This is less than ideal.

If columns haven't been initialized we should just return 0, which was
the intent of the API since the beginning.

Based on a patch by: Bastian Winkler <buz@netbuz.org>

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>