The number of argument should be retrieved by mrb_get_argc; fix #3848

You should not access `mrb->c->ci->argc` directly.

src/array.c

@@ -941,7 +941,7 @@ mrb_ary_first(mrb_state *mrb, mrb_value self)
   struct RArray *a = mrb_ary_ptr(self);
   mrb_int size, alen = ARY_LEN(a);
 
-  if (mrb->c->ci->argc == 0) {
+  if (mrb_get_argc(mrb) == 0) {
     return (alen > 0)? ARY_PTR(a)[0]: mrb_nil_value();
   }
   mrb_get_args(mrb, "|i", &size);