[Ohrrpgce] SVN: teeemcee/11408 Fix crash when thingbrowser is entered with out-of-bounds start_id and N

[subversion at HamsterRepublic.com]

teeemcee
2019-11-17 23:27:11 -0800 (Sun, 17 Nov 2019)
269
Fix crash when thingbrowser is entered with out-of-bounds start_id and NULL editor_func

It would call the NULL editor_func. This could happen if you imported a .slice
file with an out-of-range spriteset ID and then tried to browse for the
spriteset.

Reported by Sword
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U   wip/thingbrowser.bas