I wouldn't describe that as an "intentional" segfault, but rather a "natural" one, the default behaviour you get on a system with an MMU. The null case with free() is a special-case extra check explicitly inserted in the code.
NOTES
The fclose() function does not handle NULL arguments; they will result in
a segmentation violation. This is intentional - it makes it easier to make
sure programs written under FreeBSD are bug free. This behaviour is an
implementation detail, and programs should not rely upon it.