Fix `SIGSEGV` caused by GC during fiber initialization
GC may occur in the `c->stbase = mrb_malloc()` part of the `fiber_init()` function.
The `SIGSEGV` happens because it references the `c->ci->stack` field without checking `c->ci`.
This is caused by
@@ -1020,13 +1020,11 @@ gc_gray_counts(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
mrb_callinfo *ci;
if (!c || c->status == MRB_FIBER_TERMINATED) break;
+ if (!c->ci) break;
/* mark stack */
i = c->ci->stack - c->stbase;
-
- if (c->ci) {
i += mrb_ci_nregs(c->ci);
- }
if (c->stbase + i > c->stend) i = c->stend - c->stbase;
children += i;
