Fix heap corruption from wmode out-of-bound writes on windows (#6272)

* would throw error on VS2022 on GGML_FREE(wmode)
* wchar_t is usually 2 bytes, but malloc wants bytes
  * therefore `*wmode_p++ = (wchar_t)*mode;` could write off the end of the allocation
* Fixes error possibly introduced by https://github.com/ggerganov/llama.cpp/pull/6248