rBf14995aba70a46e2 fixed another error with incorrectly written CustomData,
but it also caused a problem by possibly increasing the number of layers
too much. With D15795 applied I had a crash in an old physics test file
with an invalid typemap in CustomData because of this. There may be
other issues in this area, but this seems like a way to improve the safety
of this fix anyway, unless I'm misunderstanding the original problem.