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libsec: fix bugs in tls extension handling (thanks kemal)

this patch fixes bugs in tls extension handling:

1. if conn->serverName is an empty string, tlsClientExtensions
will generate a SNI with an empty hostname, which is forbidden
according to RFC 6066:

opaque HostName<1..2^16-1>;

check if conn->serverName has at least one char.

2. checkClientExtensions fail with clients that doesn't have
extensions, because it doesn't check if ext is nil. fix that
up.

3. rewrite checkClientExtensions. some parts of the code does
not check the length properly, and it could be simplified
heavily.

--- a/sys/src/libsec/port/tlshand.c

+++ b/sys/src/libsec/port/tlshand.c

@@ -496,9 +496,7 @@

 	p = b = nil;

 	// RFC6066 - Server Name Identification

-	if(conn->serverName != nil){

-		n = strlen(conn->serverName);

-

+	if(conn->serverName != nil && (n = strlen(conn->serverName)) > 0){

 		m = p - b;

 		b = erealloc(b, m + 2+2+2+1+2+n);

 		p = b + m;

@@ -651,22 +649,20 @@

 	uchar *p, *e;

 	int i, j, n;

-	p = ext->data;

-	e = p+ext->len;

-	while(p < e){

-		if(e-p < 2)

+	if(ext == nil)

+		return 0;

+

+	for(p = ext->data, e = p+ext->len; p < e; p += n){

+		if(e-p < 4)

 			goto Short;

-		switch(get16(p)){

-		case Extec:	

-			p += 2;

-			n = get16(p);

-			if(e-p < n || n < 2)

+		p += 4;

+		if(e-p < (n = get16(p-2)))

+			goto Short;

+		switch(get16(p-4)){

+		case Extec:

+			if(n < 4 || n & 1 || get16(p) != (n -= 2))

 				goto Short;

 			p += 2;

-			n = get16(p);

-			p += 2;

-			if(e-p < n || n & 1 || n == 0)

-				goto Short;

 			for(i = 0; i < nelem(namedcurves) && c->sec->nc == nil; i++)

 				for(j = 0; j < n; j += 2)

 					if(namedcurves[i].tlsid == get16(p+j)){

@@ -673,15 +669,6 @@

 						c->sec->nc = &namedcurves[i];

 						break;

 					}

-			p += n;

-			break;

-		default:

-			p += 2;

-			n = get16(p);

-			p += 2;

-			if(e-p < n)

-				goto Short;

-			p += n;

 			break;

 		}

 	}