rtl8139: fix Pointer to local outside scope

Coverity spot:
 Assigning: iov = struct iovec [3]({{buf, 12UL},
                       {(void *)dot1q_buf, 4UL},
                       {buf + 12, size - 12}})
 (address of temporary variable of type struct iovec [3]).
 out_of_scope: Temporary variable of type struct iovec [3] goes out of scope.

Pointer to local outside scope (RETURN_LOCAL)
use_invalid:
 Using iov, which points to an out-of-scope temporary variable of type struct iovec [3].

Signed-off-by: Gonglei <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Jason Wang <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>

hw/net/rtl8139.c

@@ -1775,6 +1775,7 @@ static void rtl8139_transfer_frame(RTL8139State *s, uint8_t *buf, int size,
     int do_interrupt, const uint8_t *dot1q_buf)
 {
     struct iovec *iov = NULL;
+    struct iovec vlan_iov[3];
 
     if (!size)
     {
@@ -1789,6 +1790,9 @@ static void rtl8139_transfer_frame(RTL8139State *s, uint8_t *buf, int size,
             { .iov_base = buf + ETHER_ADDR_LEN * 2,
                 .iov_len = size - ETHER_ADDR_LEN * 2 },
         };
+
+        memcpy(vlan_iov, iov, sizeof(vlan_iov));
+        iov = vlan_iov;
     }
 
     if (TxLoopBack == (s->TxConfig & TxLoopBack))