heap problem in SemaLookup.cpp

If I build with ENABLE_EXPENSIVE_CHECKS=1 I get the following test
failure. I've attached the obvious patch to fix it.

It seems like this bug could have caused non-deterministic problems
with using-directives.


******************** TEST
FAILED! ****************

patch.semaheap (540 Bytes)

This bug could have caused a lot of problems with using-directives. Patch applied; thank you!

  - Doug