sout_std : do not crash if no destination is given

(cherry picked from commit d923971d)
Signed-off-by: Sébastien Escudier <sebastien-devel@celeos.eu>

modules/stream_out/standard.c

@@ -255,7 +255,7 @@ static int fixAccessMux( sout_stream_t *p_stream, char **ppsz_mux,
     char *psz_access = *ppsz_access;
     if( !psz_mux )
     {
-        const char *psz_ext = strrchr( psz_url, '.' );
+        const char *psz_ext = psz_url ? strrchr( psz_url, '.' ) : NULL;
         if( psz_ext )
             psz_ext++; /* use extension */
         const char *psz_mux_byext = getMuxFromAlias( psz_ext );