net: pcnet: check rx/tx descriptor ring length

The AMD PC-Net II emulator has set of control and status(CSR)
registers. Of these, CSR76 and CSR78 hold receive and transmit
descriptor ring length respectively. This ring length could range
from 1 to 65535. Setting ring length to zero leads to an infinite
loop in pcnet_rdra_addr() or pcnet_transmit(). Add check to avoid it.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/pcnet.c

@@ -1429,8 +1429,11 @@ static void pcnet_csr_writew(PCNetState *s, uint32_t rap, uint32_t new_value)
     case 47: /* POLLINT */
     case 72:
     case 74:
+        break;
     case 76: /* RCVRL */
     case 78: /* XMTRL */
+        val = (val > 0) ? val : 512;
+        break;
     case 112:
        if (CSR_STOP(s) || CSR_SPND(s))
            break;