archive/zzip: fix inverted error handler

Set the Error when zzip_seek()==-1 and not on success.  Fixes a crash
after seeking.

NEWS

@@ -1,4 +1,6 @@
 ver 0.18.19 (not yet released)
+* archive
+  - zzip: fix crash after seeking
 
 ver 0.18.18 (2014/11/18)
 * decoder

src/archive/ZzipArchivePlugin.cxx

@@ -186,12 +186,13 @@ zzip_input_seek(InputStream *is, InputPlugin::offset_type offset,
 {
 	ZzipInputStream *zis = (ZzipInputStream *)is;
 	zzip_off_t ofs = zzip_seek(zis->file, offset, whence);
-	if (ofs != -1) {
+	if (ofs < 0) {
 		error.Set(zzip_domain, "zzip_seek() has failed");
-		is->offset = ofs;
-		return true;
+		return false;
 	}
-	return false;
+
+	is->offset = ofs;
+	return true;
 }
 
 /* exported structures */