gsocket: test if family is <= 0, not < 0

gsocket: test if family is <= 0, not < 0

If all members of GSocketFamily are supported on the platform, then
all of its values will be positive, and so the enum might become
unsigned, in which case testing for "family < 0" might cause warnings.
But we want to return an error if family == 0 (aka
G_SOCKET_FAMILY_INVALID) anyway, so just tweak the test accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=674592