Add `NUL` always to short strings; ref
MODIFY
src/string.c
src/string.c
@@ -594,6 +594,7 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
RSTR_SET_EMBED_FLAG(s1);
memcpy(s1->as.ary, RSTR_PTR(s2), len);
RSTR_SET_EMBED_LEN(s1, len);
+ RSTR_PTR(s1)[len] = '\0';
}
else {
str_make_shared(mrb, s2, s1);

Add `NUL` always to short strings; ref 98fc887