rxrpc: out of bound read in debug code

Smatch complains because we are using an untrusted index into the
rxrpc_acks[] array.  It's just a read and it's only in the debug code,
but it's simple enough to add a check and fix it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>