I'd appreciate it if someone more familiar with kfreebsd than me would have a look and see if they can understand better what's behind the failure.
It seems that it builds fine under 8.3 and 9.0 kernels from wheezy, whileit fails under 8.1 kernel from squeeze in otherwise same (kvm, almost wheezy) kfreebsd-i386 environment.
The "export MALLOC_CHECK_=7" generates failure under 8.1 kernel reliable. I hope that buildd kernels will be upgraded to 9.0 after wheezy release. Petr kfreebsd-image-8.1-1-686 8.1+dfsg-8+squeeze3 kfreebsd-image-8.3-1-686 8.3-6 kfreebsd-image-9.0-2-686 9.0-10