Fix T102893: Assert Opening File Browser (Win32)

Fix debug assert opening File Browser on Windows platform.

See D16672 for more details.

Differential Revision: https://developer.blender.org/D16672

Reviewed by Julian Eisel

source/blender/editors/space_file/filelist.cc

@@ -2942,7 +2942,7 @@ static char *current_relpath_append(const FileListReadJob *job_params, const cha
     return BLI_strdup(filename);
   }
 
-  BLI_assert(relbase[strlen(relbase) - 1] == SEP);
+  BLI_assert(ELEM(relbase[strlen(relbase) - 1], SEP, ALTSEP));
   BLI_assert(BLI_path_is_rel(relbase));
 
   char relpath[FILE_MAX_LIBEXTRA];