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ebml: use -1 to specify unknown size; skip unknown elements properly

--- a/ebml.c

+++ b/ebml.c

@@ -343,8 +343,8 @@

 		v |= c;

 		if(v & m){

 			*out = isid ? v : (v & ~m);

-			if(!isid && *out == (1<<(7*n))-1)

-				*out = 0;

+			if(!isid && *out == (1<<(7*n))-1) /* unknown size */

+				*out = -1;

 			return n;

 		}

 		if(!isid)

@@ -429,18 +429,24 @@

 		werrstr("id: %r");

 		return -1;

 	}

-	if((s = elspec(x)) != nil)

+	if((s = elspec(x)) != nil){

 		*el = *s;

-	else

-		memset(el, 0, sizeof(*el));

-	el->id = x;

-	r = ebmluint(f, sz-n, &x);

-	n += r;

-	if(n >= sz || r < 0 || sz-n < x){

-		werrstr("sz: (sz=%lld n=%lld r=%lld x=%lld): %r", sz, n, r, x);

-		return -1;

+		el->id = x;

+		r = ebmluint(f, sz-n, &x);

+		n += r;

+		if(n >= sz || r < 0 || sz-n < x){

+			werrstr("sz: (sz=%lld n=%lld r=%lld x=%lld): %r", sz, n, r, x);

+			return -1;

+		}

+		*esz = x;

+	}else{ /* skip invalid elements */

+		while(--n > 0)

+			Bungetc(f);

+		el->id = x;

+		el->type = -1;

+		*esz = 0;

+		return 0;

 	}

-	*esz = x;

 	return n;

 }