[efi] Guard against GetStatus() failing to return a NULL TX buffer

The UEFI specification requires the EFI_SIMPLE_NETWORK_PROTOCOL
GetStatus() method to set TxBuf to NULL if there are no transmit
buffers to recycle.

Some implementations (observed with Lan9118Dxe in EDK2) fill in TxBuf
only when there is a transmit buffer to recycle, which leads to large
numbers of "spurious TX completion" errors.

Work around this problem by initialising TxBuf to NULL before calling
the GetStatus() method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/efi/snpnet.c

 	int rc;
 
 	/* Get status */
+	txbuf = NULL;
 	if ( ( efirc = snp->snp->GetStatus ( snp->snp, &irq, &txbuf ) ) != 0 ) {
 		rc = -EEFI ( efirc );
 		DBGC ( snp, "SNP %s could not get status: %s\n",