main/frame: Don't report empty disallow values as an error

In realtime, it is normal to have a database with both 'allow' and 'disallow'
columns in the schema. It is perfectly valid to have an 'allow' value of
'!all,g722,ulaw,alaw' and no 'disallow' value. Unlike in static conf files,
you can't *not* provide the disallow value. Thus, the empty disallow value
causes a spurious WARNING message, which is kind of annoying.

This patch makes it so that a 'disallow' value with no ... value ... is
ignored. Granted, you can still screw this up as well, as technically
specifying 'disallow=all,!ulaw' allows only ulaw, and then you would have no
'allow' value in your database. But really, why would you do that? WHY?

ASTERISK-16779 #close
Reported by: Atis Lezdins
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Merged revisions 432970 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 432971 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432972 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/format_cap.c

@@ -319,6 +319,10 @@ int ast_format_cap_update_by_allow_disallow(struct ast_format_cap *cap, const ch
 	int res = 0, all = 0, iter_allowing;
 	char *parse = NULL, *this = NULL, *psize = NULL;
 
+	if (!allowing && ast_strlen_zero(list)) {
+		return 0;
+	}
+
 	parse = ast_strdupa(list);
 	while ((this = strsep(&parse, ","))) {
 		int framems = 0;