media formats: update res_format_attr_opus & silk

In r419044, we changed how formats were handled, but the return value
of the format_parse_sdp_fmtp functions in res_format_attr_opus and
res_format_attr_silk were not updated, causing calls to fail.  Ran
into this when getting codec_opus working with Asterisk 13.

Once the return value was corrected, we were crashing in opus_getjoint
because of NULL format attributes.  I've fixed this as well in this
patch.

Review: https://reviewboard.asterisk.org/r/4371/
........

Merged revisions 431267 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431268 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_format_attr_opus.c

@@ -51,6 +51,12 @@ struct opus_attr {
 	unsigned int spropstereo;	        /* Default 0 */
 };
 
+static struct opus_attr default_opus_attr = {
+	.fec    = 0,
+	.dtx    = 0,
+	.stereo = 0,
+};
+
 static void opus_destroy(struct ast_format *format)
 {
 	struct opus_attr *attr = ast_format_get_attribute_data(format);
@@ -120,7 +126,7 @@ static struct ast_format *opus_parse_sdp_fmtp(const struct ast_format *format, c
 		attr->dtx = val;
 	}
 
-	return 0;
+	return cloned;
 }
 
 static void opus_generate_sdp_fmtp(const struct ast_format *format, unsigned int payload, struct ast_str **str)
@@ -163,6 +169,14 @@ static struct ast_format *opus_getjoint(const struct ast_format *format1, const
 	struct ast_format *jointformat;
 	struct opus_attr *attr_res;
 
+	if (!attr1) {
+		attr1 = &default_opus_attr;
+	}
+
+	if (!attr2) {
+		attr2 = &default_opus_attr;
+	}
+
 	jointformat = ast_format_clone(format1);
 	if (!jointformat) {
 		return NULL;

res/res_format_attr_silk.c

@@ -94,7 +94,7 @@ static struct ast_format *silk_parse_sdp_fmtp(const struct ast_format *format, c
 		attr->fec = val;
 	}
 
-	return 0;
+	return cloned;
 }
 
 static void silk_generate_sdp_fmtp(const struct ast_format *format, unsigned int payload, struct ast_str **str)