Land rapid7#8737, better handle sudden disconnects with SMTP servers

Author: Brent Cook

lib/msf/core/exploit/smtp_deliver.rb

@@ -184,7 +184,7 @@ def send_message(data)
 
     raw_send_recv("MAIL FROM: <#{mailfrom}>\r\n", nsock)
     res = raw_send_recv("RCPT TO: <#{mailto}>\r\n", nsock)
-    if res[0..2] == '250'
+    if res && res[0..2] == '250'
       resp = raw_send_recv("DATA\r\n", nsock)
 
       # If the user supplied a Date field, use that, else use the current
@@ -242,10 +242,12 @@ def raw_send_recv(cmd, nsock=self.sock)
       # to dump it all.
       vprint_status("C: #{((cmd.length > 120) ? cmd[0,120] + "..." : cmd).strip}")
     end
-
-    nsock.put(cmd)
-    res = nsock.get_once
-
+    begin
+      nsock.put(cmd)
+      res = nsock.get_once
+    rescue
+      return nil
+    end
     # Don't truncate the server output because it might be helpful for
     # debugging.
     vprint_status("S: #{res.strip}") if res