Handle case where an unknown format is used to get the preferred codec size

In ast_codec_pref_getsize, if an unknown format is passed to the method,
no preferred codec will be selected and a negative number will be used to
index into the format list.  The method now logs an unknown format as a
warning, and returns an empty format list.

(issue ASTERISK-19655)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1863/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@362368 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/frame.c

@@ -1195,6 +1195,11 @@ struct ast_format_list ast_codec_pref_getsize(struct ast_codec_pref *pref, forma
 		}
 	}
 
+	if (idx < 0) {
+		ast_log(AST_LOG_WARNING, "Format %s unknown; unable to get preferred codec packet size\n", ast_getformatname(format));
+		return fmt;
+	}
+
 	for (x = 0; x < ARRAY_LEN(AST_FORMAT_LIST); x++) {
 		if (pref->order[x] == (idx + 1)) {
 			framems = pref->framing[x];