fix(upgrade): guard mainfile template reads

Handle unreadable mainfile.dist.php explicitly in Upgrade_230::write_mainfile() so the upgrader fails cleanly instead of falling through into noisier warnings.

Author: mambax7

upgrade/upd-2.0.18-to-2.3.0/index.php

@@ -417,6 +417,11 @@ public function write_mainfile($vars)
         $file = __DIR__ . '/mainfile.dist.php';
 
         $lines = file($file);
+        if (false === $lines) {
+            echo ERR_COULD_NOT_WRITE_MAINFILE;
+
+            return false;
+        }
         foreach (array_keys($lines) as $ln) {
             if (preg_match("/(define\()([\"'])(XOOPS_[^\"']+)\\2,\s*([0-9]+)\s*\)/", $lines[$ln], $matches)) {
                 if (isset($vars[$matches[3]])) {