multifilesrc: fix regression with starting from index set via index property
multifilesrc: fix regression with starting from index set via index property

When we haven't started yet, set the start_index when we set the index property,
so that we start at the right index position after the initial seek. The index
property was never really meant to be for writing, but it used to work, so let's
support it for backwards compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=739472

StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,rawoul/gst-plugins-good,sebras/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good