soup_uri_new: do not allow invalid URIs to be returned, except via soup_uri_new (NULL)
soup_uri_new: do not allow invalid URIs to be returned, except via soup_uri_new (NULL)

Also document the possible NULL return.

Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=667637
Bug-NB: NB#294977

jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,Distrotech/libsoup,fanc999/libsoup,Distrotech/libsoup,jwendell/libsoup,Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup,fanc999/libsoup,Distrotech/libsoup