res_pjsip: Validate that contact uris start with sip: or sips:

Currently we use pjsip_parse_hdr to validate contact uris but it
appears that it allows uris without a scheme if there's a port
supplied.  I.E myexample.com will fail but myexample.com:5060 will
pass even though it has no scheme.  This causes SEGVs later on
whenever the uri is used.

To prevent this, permanent_contact_validate has been updated to check
that the scheme is either 'sip' or 'sips'.

2 uses of possibly-null endpoint have also been fixed in
create_out_of_dialog_request.

ASTERISK-24999

Change-Id: Ifc17d16a4923e1045d37fe51e43bbe29fa556ca2
Reported-by: Brad Latus