chan_iax2: Fix a crash when using chan_iax2 jitterbuffer settings

Caused by format changes in Asterisk 13

ASTERISK-24265 #close
Reported by: Dafi Ni
Review: https://reviewboard.asterisk.org/r/3999/
........

Merged revisions 423524 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423526 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_iax2.c

@@ -4123,7 +4123,7 @@ static void __get_from_jb(const void *p)
 	if(ms >= (next = jb_next(pvt->jb))) {
 		struct ast_format *voicefmt;
 		voicefmt = ast_format_compatibility_bitfield2format(pvt->voiceformat);
-		ret = jb_get(pvt->jb, &frame, ms, ast_format_get_default_ms(voicefmt));
+		ret = jb_get(pvt->jb, &frame, ms, voicefmt ? ast_format_get_default_ms(voicefmt) : 20);
 		switch(ret) {
 		case JB_OK:
 			fr = frame.data;