pg_crypto failures with llvm on OSX

Buildfarm member mussel (OS X 10.7.3, llvm-gcc 4.2.1, x86_64)seems to be 
getting consistent warnings when running the pgcrypto regression tests, 
that look like this:
   WARNING: detected write past chunk end in ExprContext 0x7fec2b11eb58

Does anyone have an idea why that might be?

cheers

andrew