Do not generate a SDP file if SDP is not yet defined (fixes: #2981)

(cherry picked from commit 96f5cc4d)

modules/stream_out/rtp.c

@@ -1401,6 +1401,9 @@ static int FileSetup( sout_stream_t *p_stream )
     sout_stream_sys_t *p_sys = p_stream->p_sys;
     FILE            *f;
 
+    if( p_sys->psz_sdp == NULL )
+        return VLC_EGENERIC; /* too early */
+
     if( ( f = utf8_fopen( p_sys->psz_sdp_file, "wt" ) ) == NULL )
     {
         msg_Err( p_stream, "cannot open file '%s' (%m)",