wifi: rtl818x: rtl8187: Fix potential buffer underflow in rtl8187_rx_cb()

In the Linux kernel, the following vulnerability has been resolved: wifi: rtl818x: rtl8187: Fix potential buffer underflow in rtl8187_rx_cb() The rtl8187_rx_cb() calculates the rx descriptor header address by subtracting its size from the skb tail pointer. However, it does not validate if the received packet (skb->len from urb->actual_length) is large enough to contain this header. If a truncated packet is received, this will lead to a buffer underflow, reading memory before the start of the skb data area, and causing a kernel panic. Add length checks for both rtl8187 and rtl8187b descriptor headers before attempting to access them, dropping the packet cleanly if the check fails.