session: fix crash when storing view-source URI without host component
Fixes, for example:
epiphany-CRITICAL **: 10:29:44.752: Refusing to save session due to invalid URL view-source:file:///home/mcatanzaro/a.html
I'm not sure it's actually a good idea to store view-source URIs in the session state, but let's just make a small change to handle it gracefully instead of rethinking that.
Fixes #1987 (closed)