dest.c: Call cupsEnumDests() only if data.name is not NULL (fixes #719)

dest.c: Call cupsEnumDests() only if data.name is not NULL (fixes #719)

Fixes regression created by #452 - in case there is no default
destination and `cupsGetNamedDest()` is called to get one (by calling it
with argument `name` as NULL), the function crashes.

It happens because we try to look for the default printer on the network
(where we access `data.name`, which is NULL, in callback), but we never
found out the printer's name.

Original fix by Emilio Cobos Alvarez.