mailparse: Fix incorrect parsing of some multipart messages

This patch drops the content after the last boundary in a multi-part message
which prevents it from being considered in some case where the available
content sections are not favorable.

Backport me to v1.9

include/pear/Mail/mimeDecode.php

@@ -529,6 +529,15 @@ class Mail_mimeDecode extends PEAR
         if ($input instanceof StringView) {
             $parts = $input->split('--' . $boundary);
             array_shift($parts);
+
+            if (count($parts) > 0
+                && $parts[count($parts)-1]->substr(0, 2)->__toString() == '--'
+            ) {
+                // Drop the last part if it starts with '--' as such would
+                // be past the end of a multipart section
+                array_pop($parts);
+            }
+
             return $parts;
         }