Guard rb_obj_embedded_size for zero fields

With smaller pool sizes (e.g. 32 bytes), an RObject with 0 embedded
fields would request a size smaller than the minimum meaningful object.
Ensure at least 1 field worth of space so the embedded size is always
large enough for a valid RObject.

Author: eightbitraptor

internal/object.h

@@ -64,6 +64,7 @@ RBASIC_SET_CLASS(VALUE obj, VALUE klass)
 static inline size_t
 rb_obj_embedded_size(uint32_t fields_count)
 {
+    if (fields_count < 1) fields_count = 1;
     return offsetof(struct RObject, as.ary) + (sizeof(VALUE) * fields_count);
 }
 #endif /* INTERNAL_OBJECT_H */