wifi: rtw89: fix leak in rtw89_core_send_nullfunc()

If there is no rtwsta_link found in rtw89_core_send_nullfunc(), allocated
skb is leaked.  Free it on the error handling path.

Found by Linux Verification Center (linuxtesting.org).

Fixes: a8ba4aca ("wifi: rtw89: send nullfunc based on the given link")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250919210852.823912-4-pchelkin@ispras.ru