Don't disable audio if V4L video dev does not have an audio dev too. Closes:#254

Thanks to Timothy Lee < timothy d0t lee @t siriushk d0t c0m > for the patch.

modules/access/v4l/v4l.c

@@ -900,7 +900,7 @@ static int OpenVideoDev( demux_t *p_demux, char *psz_device )
         msg_Dbg( p_demux, "invalid channel, falling back on channel 0" );
         p_sys->i_channel = 0;
     }
-    if( p_sys->i_audio >= p_sys->vid_cap.audios )
+    if( p_sys->vid_cap.audios && p_sys->i_audio >= p_sys->vid_cap.audios )
     {
         msg_Dbg( p_demux, "invalid audio, falling back with no audio" );
         p_sys->i_audio = -1;