fix problem with dtmf on e&m (issue #6364)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@8729 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_zap.c

@@ -5289,7 +5289,7 @@ static void *ss_thread(void *data)
 			else 
 				ast_dsp_digitmode(p->dsp,DSP_DIGITMODE_DTMF | p->dtmfrelax);
 		}
-		dtmfbuf[0] = 0;
+		memset(dtmfbuf, 0, sizeof(dtmfbuf));
 		/* Wait for the first digit only if immediate=no */
 		if (!p->immediate)
 			/* Wait for the first digit (up to 5 seconds). */