soup-session-feature: work around an old Eclipse bug

Add g_return_if_fail() checks to soup_session_feature_attach() and
soup_session_feature_detach(), to work around a bug in Eclipse that
was fixed a long time ago
(https://bugs.eclipse.org/bugs/show_bug.cgi?id=404776) but which
people keep reporting against libsoup.

https://bugzilla.gnome.org/show_bug.cgi?id=710242