Fix potential segfault when flat images has changed on netcode

A miss on !2497 (merged) where the fields on SD_FLOORPIC and SD_CEILPIC weren't loaded in the usual way, which ended up applying the old logic of pushing the window further rather than using the buffer that stores save information. This caused it to read arbitrary data, and would often trigger segfaults.

I sure hope I don't get fired for that blunder

Eh, shit happens. Let's just fix it and get it over with.

approved this merge request

added Netcode Regression labels

changed milestone to %2.2.14

merged

mentioned in commit 64d84ec4