bugfix in hl_buffer_str_sub which might cause input string to be gc'ed

src/std/buffer.c

@@ -65,6 +65,7 @@ static void buffer_append_new( hl_buffer *b, const uchar *s, int len ) {
 
 HL_PRIM void hl_buffer_str_sub( hl_buffer *b, const uchar *s, int len ) {
 	stringitem it;
+	int offset = 0;
 	if( s == NULL || len <= 0 )
 		return;
 	b->totlen += len;
@@ -78,11 +79,11 @@ HL_PRIM void hl_buffer_str_sub( hl_buffer *b, const uchar *s, int len ) {
 		} else {
 			memcpy(it->str + it->len,s,free<<1);
 			it->len += free;
-			s += free;
+			offset = free;
 			len -= free;
 		}
 	}
-	buffer_append_new(b,s,len);
+	buffer_append_new(b,s + offset,len);
 }
 
 HL_PRIM void hl_buffer_str( hl_buffer *b, const uchar *s ) {