libosmo-mgcp: cleanup audio codex alloc

No need to complicate audio codes with pointers, our "usual" string is
barely larger than a poointer, five times smaller than a talloc header,
and most importantly not really optional anyway...

Change-Id: Icc41643050a5e1ca3c66f307d60b6911ba1b8032
diff --git a/include/osmocom/mgcp/mgcp_network.h b/include/osmocom/mgcp/mgcp_network.h
index d6d90dd..b9cf5e3 100644
--- a/include/osmocom/mgcp/mgcp_network.h
+++ b/include/osmocom/mgcp/mgcp_network.h
@@ -81,8 +81,8 @@
 	uint32_t frame_duration_den;
 
 	int payload_type;
-	char *audio_name;
-	char *subtype_name;
+	char audio_name[64];
+	char subtype_name[64];
 
 	bool param_present;
 	struct mgcp_codec_param param;