Duke3D: Fix default SE26 speed if no GPSPEED tile is present.

Explanation: The default sector extra is set to 256 in prelevel(), and is not 0.

Fixes issue with "City of Screams" map mentioned here:

https://forums.duke4.net/topic/11076-new-swinging-door-behavoir-test/page__view__findpost__p__357746

Tested with "City of Screams" and all official Ion Fury maps that make use of SE26.