assert() is too harsh here I think, use SAL_WARN_IF() instead

I run into the assertion now in TiledLibreOffice, but if I drop the
assert(), it still works. So let's just print a warning (that
everybody will cheerfully ignore, ha ha) instead. I have no idea what
this code actually is doing...

(Originally the code had DBG_ASSERT(), which despite its name also
just prints a warning, which I changed to a real assert() in
8293b29304716d6bd117ab233b1238e1385ae592 for some reason.)

Change-Id: I928078aaf3648c8d04c59f271504c7d3b02e1bb5