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libauth: fix out of bounds memory access in _parseattr()

empty token would read ""[-1] accidentally in the AttrQuery case.

--- a/libauth/attr.c

+++ b/libauth/attr.c

@@ -128,7 +128,7 @@

 _parseattr(char *s)

 {

 	char *p, *t, *tok[256];

-	int i, ntok, type;

+	int i, ntok;

 	Attr *a;

 	s = strdup(s);

@@ -139,25 +139,17 @@

 	a = nil;

 	for(i=ntok-1; i>=0; i--){

 		t = tok[i];

-		if(p = strchr(t, '=')){

+		if((p = strchr(t, '=')) != nil){

 			*p++ = '\0';

-		//	if(p-2 >= t && p[-2] == ':'){

-		//		p[-2] = '\0';

-		//		type = AttrDefault;

-		//	}else

-				type = AttrNameval;

-			a = _mkattr(type, t, p, a);

-			setmalloctag(a, getcallerpc(&s));

-		}

-		else if(t[strlen(t)-1] == '?'){

-			t[strlen(t)-1] = '\0';

+			a = _mkattr(AttrNameval, t, p, a);

+		}else if((p = strchr(t, '\0')-1) >= t && *p == '?'){

+			*p = '\0';

 			a = _mkattr(AttrQuery, t, "", a);

-			setmalloctag(a, getcallerpc(&s));

 		}else{

 			/* really a syntax error, but better to provide some indication */

 			a = _mkattr(AttrNameval, t, "", a);

-			setmalloctag(a, getcallerpc(&s));

 		}

+		setmalloctag(a, getcallerpc(&s));

 	}

 	free(s);

 	return cleanattr(a);