string.c: avoid integer type mixture in mrb_str_cat().
@@ -2632,7 +2632,7 @@ mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len)
{
struct RString *s = mrb_str_ptr(str);
mrb_int capa;
- size_t total;
+ mrb_int total;
ptrdiff_t off = -1;
if (len == 0) return str;
