res_pjsip_dtmf_info: NULL terminate the message body.

PJSIP does not ensure that when printing the message body the
buffer will be NULL terminated. This is problematic when searching
for the signal and duration values of the DTMF.

This change ensures the buffer is always NULL terminated.

Change-Id: I52653a1a60c93092d06af31a27408d569cc98968