Add API call to determine if format capability structure is "empty".

Empty here means that there are no formats in the format_cap structure
or the only format in it is the "none" format.

I've added calls to check the emptiness of a format_cap in a few places
in order to short-circuit operations that would otherwise be pointless
as well as to prevent some assertions from being triggered in cases
where channels with no formats are used.



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@423414 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -5327,6 +5327,11 @@ static int set_format(struct ast_channel *chan, struct ast_format_cap *cap_set,
 	ast_assert(rawformat != NULL);
 
 	cap_native = ast_channel_nativeformats(chan);
+	if (ast_format_cap_empty(cap_native)) {
+		ast_log(LOG_ERROR, "Unable to set format because channel %s supports no formats\n",
+				ast_channel_name(chan));
+		return -1;
+	}
 
 	/* Find a translation path from the native format to one of the desired formats */
 	if (!direction) {