rrunner: fix buffer overflow

tx_skbuff is define as:
        struct sk_buff          *tx_skbuff[TX_RING_ENTRIES];

EVT_RING_ENTRIES is 64 and TX_RING_ENTRIES is 32.

This function is in a error path so that's why it wasn't noticed.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>