fix crash generating mdn receipt when original message has no message-id sr=mscott 224738

mailnews/extensions/mdn/src/nsMsgMdnGenerator.cpp

@@ -598,12 +598,14 @@ nsresult nsMsgMdnGenerator::CreateFirstPart()
     m_headers->ExtractHeader(HEADER_MESSAGE_ID, PR_FALSE,
                              getter_Copies(m_messageId));
     
+    if (!m_messageId.IsEmpty())
+    {
       if (*m_messageId.get() == '<')
           tmpBuffer = PR_smprintf("References: %s" CRLF, m_messageId.get());
       else
           tmpBuffer = PR_smprintf("References: <%s>" CRLF, m_messageId.get());
       PUSH_N_FREE_STRING(tmpBuffer);
-
+    }
     tmpBuffer = PR_smprintf("%s" CRLF, "MIME-Version: 1.0");
     PUSH_N_FREE_STRING(tmpBuffer);