[Ohrrpgce] SVN: teeemcee/11782 Fix crash in backdrop editor if .mxs lump missing in old games

[subversion at HamsterRepublic.com]

teeemcee
2020-04-27 18:32:36 -0700 (Mon, 27 Apr 2020)
232
Fix crash in backdrop editor if .mxs lump missing in old games

SAMPLE.RPG from 1998 (and maybe newer) has no .mxs lump. It got upgraded to
a backdrops.rgfx containing 0 backdrops, which the editor couldn't handle

Crashrpt 3e9a90db
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U   wip/drawing.bas
U   wip/loading.rbas