Corrects the RTP DTMF buffering management for the following case: when a key is hold down on Windows, it generates several start DTMF events and only one stop when released.

git-svn-id: https://svn.java.net/svn/jitsi~svn/libjitsi@10657 198c5059-ef06-4ec0-9a75-fdd6b0ef00a6