sh: Assert INTOFF rather than applying it in ck*

As I noted in https://reviews.freebsd.org/D22756, INTOFF should be in effect
when calling ckmalloc/ckrealloc/ckfree to avoid memory leaks and double
frees. Therefore, change the functions to check if INTOFF is in effect
instead of applying it.

Reviewed by: bdrewery
Differential Revision: https://reviews.freebsd.org/D24599