Fix fields object in embedded struct

We don't set RSTRUCT_GEN_FIELDS when RCLASS_MAX_IV_COUNT(klass) != 0, so
we need to set RSTRUCT_SET_FIELDS_OBJ to 0 otherwise it may have an
invalid value and crash.

Author: peterzhu2118

struct.c

@@ -826,12 +826,17 @@ struct_alloc(VALUE klass)
         }
 
         NEWOBJ_OF(st, struct RStruct, klass, flags, embedded_size, 0);
-        if (RCLASS_MAX_IV_COUNT(klass) == 0
-            && !rb_shape_obj_has_fields((VALUE)st)
-            && embedded_size < rb_gc_obj_slot_size((VALUE)st)) {
-            FL_UNSET_RAW((VALUE)st, RSTRUCT_GEN_FIELDS);
+        if (RCLASS_MAX_IV_COUNT(klass) == 0) {
+            if (!rb_shape_obj_has_fields((VALUE)st)
+                    && embedded_size < rb_gc_obj_slot_size((VALUE)st)) {
+                FL_UNSET_RAW((VALUE)st, RSTRUCT_GEN_FIELDS);
+                RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
+            }
+        }
+        else {
             RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
         }
+
         rb_mem_clear((VALUE *)st->as.ary, n);
 
         return (VALUE)st;