When the number of elements is zero, don't malloc 32GB on 64-bit systems.

Fixes unexpected failures on FreeBSD/amd64 of:
  CFrontend/2005-09-24-BitFieldCrash.c:
  CFrontend/2007-02-04-EmptyStruct.c:
  CFrontend/2007-03-26-ZeroWidthBitfield.c:
  CodeGen/Generic/2005-10-18-ZeroSizeStackObject.ll:

llvm-svn: 35828