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libc/idn: clean up reliance on questionable corners of seprint behavior

--- a/sys/src/libc/9sys/idn.c

+++ b/sys/src/libc/9sys/idn.c

@@ -211,12 +211,11 @@

 		if(cistrncmp(cp, "xn--", 4) == 0)

 			if((nr = punydecode(nc-4, cp+4, nelem(rb), rb)) < 0)

 				return -1;

-		if((dp = seprint(dp, de, "%.*S", nr, rb)) == nil)

-			return -1;

-		if(dp >= de)

-			return -1;

+		dp = seprint(dp, de, "%.*S", nr, rb);

 		if(cp[nc] == 0)

 			break;

+		if(dp + 1 == de)

+			return -1;

 		*dp++ = '.';

 		cp += nc+1;

 	}

@@ -256,9 +255,9 @@

 			if((dp = seprint(dp, de, "%.*s", nc, cp)) == nil)

 				return -1;

 		}else{

-			if((dp = seprint(dp, de, "xn--")) == nil)

-				return -1;

-			if((n = punyencode(nr, rb, de - dp, dp)) < 0)

+			dp = seprint(dp, de, "xn--");

+			n = punyencode(nr, rb, de - dp, dp);

+			if(n < 0 || dp+n == de)

 				return -1;

 			dp += n;

 		}

-- 

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