Fix build of amd64-gcc6 and i386-gcc6 with LLVM 11.

[jhb]

Summary

clang errors when assigning NULL (nullptr) to a bool.

[jhb]

@zeising, would you be able to test this in your existing LLVM 11 poudriere? I've tested it on a stable/12 poudriere so I know it still builds with clang 10, and this was the error I found in the fallout logs.

[zeising]

both freebsd-gcc6@amd64 and @i386 builds fine on amd64, @i386 also builds fine on i386. I didn't try @amd64 on i386, but I see no reason it shouldn't work.