Add a missing asan_unpoisoning_p in gc_set_candidate_object_i

It walks the heap, and checks for T_NONE and T_ZOMBIE objects, so it
needs to unpoison these slots before accessing them when ASAN is
enabled.

gc.c

@@ -9362,6 +9362,7 @@ gc_set_candidate_object_i(void *vstart, void *vend, size_t stride, void *data)
     rb_objspace_t *objspace = &rb_objspace;
     VALUE v = (VALUE)vstart;
     for (; v != (VALUE)vend; v += stride) {
+        asan_unpoisoning_object(v) {
             switch (BUILTIN_TYPE(v)) {
             case T_NONE:
             case T_ZOMBIE:
@@ -9377,6 +9378,7 @@ gc_set_candidate_object_i(void *vstart, void *vend, size_t stride, void *data)
                 }
             }
         }
+    }
 
     return 0;
 }