Prevent Echo() from relaying control, null, and modem frames

Echo()'s description states that it echoes audio, video, and DTMF except for #
while it actually echoes any frame that it receives other than DTMF #.  This
was causing frame storms in the test suite in some circumstances where Echo()
was attached to both ends of a pair of local channels and control frames
were being periodically generated.  Echo()'s behavior and description have
been modifed so that it only echoes media and non-# DTMF frames.
........

Merged revisions 360033 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 360034 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@360036 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_echo.c

@@ -40,12 +40,13 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 /*** DOCUMENTATION
 	<application name="Echo" language="en_US">
 		<synopsis>
-			Echo audio, video, DTMF back to the calling party
+			Echo media, DTMF back to the calling party
 		</synopsis>
 		<syntax />
 		<description>
-			<para>Echos back any audio, video or DTMF frames read from the calling 
-			channel back to itself. Note: If '#' detected application exits</para>
+			<para>Echos back any media or DTMF frames read from the calling 
+			channel back to itself. This will not echo CONTROL, MODEM, or NULL
+			frames. Note: If '#' detected application exits.</para>
 			<para>This application does not automatically answer and should be
 			preceeded by an application such as Answer() or Progress().</para>
 		</description>
@@ -70,7 +71,10 @@ static int echo_exec(struct ast_channel *chan, const char *data)
 		}
 		f->delivery.tv_sec = 0;
 		f->delivery.tv_usec = 0;
-		if (ast_write(chan, f)) {
+		if (f->frametype != AST_FRAME_CONTROL
+			&& f->frametype != AST_FRAME_MODEM
+			&& f->frametype != AST_FRAME_NULL
+			&& ast_write(chan, f)) {
 			ast_frfree(f);
 			goto end;
 		}