Prevent warnings with some HTML attachments.

HTML attachments may be rewritten on download to adjust links to images
that are stored in RT as separate attachments.  The function that does
the rewrite may return undef if there is nothing to rewrite, causing a
'Use of uninitialized value' warning.

share/html/Ticket/Attachment/dhandler

@@ -83,6 +83,7 @@ elsif (lc $content_type eq 'text/html') {
         Content     => \$content,
         Attachment  => $AttachmentObj,
     );
+    $count //= 0;  # RewriteInlineImages can return undef when nothing needs to be rewritten
     RT->Logger->debug("Rewrote $count CID images when displaying original HTML attachment #$attach");
 }