contrib/rtp: Fix default payload case

There is the wrong record field selection being used to extract the
default value. It returns the tuple offset instead of the value.

This patch fixes this.

Sponsored-by: On-Waves ehf

openbsc/contrib/rtp/gen_rtp_header.erl

@@ -45,7 +45,9 @@ main([First | RemArgs], Opts) ->
 	InFile = proplists:get_value(file, Opts, undef),
 
         Payload = case {PayloadData, InFile} of
-	    {undef, undef} -> #rtp_packet.payload;
+	    {undef, undef} ->
+		% use default value
+		#rtp_packet{}#rtp_packet.payload;
 	    {P, undef} -> P;
 	    {_, File} ->
 		log(info, "Loading file '~s'~n", [File], Opts),