Fix harmless warnings in quic_encode()
Fix harmless warnings in quic_encode()

The quic code has been changed recently this way:

-    ASSERT(encoder->usr, line);
+    if (line == NULL) {
+        spice_warn_if_reached();
+        return QUIC_ERROR;
+    }

It appears that the only caller of quic_encode() gives a NULL line and
rely on the more_lines() callback to return new lines instead.

Adjust the code accordingly, adding a few more checks to verify the
caller gives/returns correct values.

Fantu/spice-common,SPICE/spice-common,Fantu/spice-common,fgouget/spice-common,fgouget/spice-common,SPICE/spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,fgouget/spice-common,Fantu/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,elmarco/spice-common,elmarco/spice-common,elmarco/spice-common