avimux: Do not write index and header if idx is NULL
avimux: Do not write index and header if idx is NULL

Fixes criticals with e.g.
videotestsrc num-buffers=1 ! identity drop-probability=1.0 ! avimux ! fakesink

https://bugzilla.gnome.org/show_bug.cgi?id=748700

Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good