Since `callinfo::blk` is not marked in the GC, it may be reclaimed in
the sweep phase. When it reclaimed, it will become either (a) a non Proc
object, or (b) a Proc object that happen to have the same address.
For case (a), adding `b->tt == MRB_TT_PROC` check works. We should avoid
the following `MRB_PROC_STRICT_P()` and `MRB_PROC_ENV()` operations for
non Proc objects.
For case (b), `MRB_PROC_ENV(b) == CI_ENV(&c->ci[-1])` check should work.
Unrelated Proc objects should be filtered by the check.
We don't need to clear `callinfo::blk` by `NULL` because the callinfo
struct will be discarded afterward in the `cipop()` function.
Yukihiro "Matz" Matsumoto
81f98a3925