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wifi: fix recvbeacon()

we used to read beyond the boundaries of the becon because of
the end pointer was offset by the beacon header. this is
also what caused the double entries.

--- a/sys/src/9/pc/wifi.c

+++ b/sys/src/9/pc/wifi.c

@@ -372,10 +372,10 @@

 recvbeacon(Wifi *wifi, Wnode *wn, uchar *d, int len)

 {

 	static uchar wpa1oui[4] = { 0x00, 0x50, 0xf2, 0x01 };

-	uchar *e, *x, *p;

-	uchar t, m[256/8];

+	uchar *e, *x, *p, t;

-	if(len < 8+2+2)

+	len -= 8+2+2;

+	if(len < 0)

 		return;

 	d += 8;	/* timestamp */

@@ -384,19 +384,12 @@

 	wn->cap = d[0] | d[1]<<8;

 	d += 2;

-	memset(m, 0, sizeof(m));

 	for(e = d + len; d+2 <= e; d = x){

 		d += 2;

 		x = d + d[-1];

-		if(x > e)

+		if(x > e)			

 			break;	/* truncated */

 		t = d[-2];

-

-		/* skip double entries */

-		if(m[t/8] & 1<<(t%8))

-			continue;

-		m[t/8] |= 1<<(t%8);

-

 		switch(t){

 		case 0:		/* SSID */

 			len = 0;

-- 

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