gstindex: Make sure writer is non-NULL.
gstindex: Make sure writer is non-NULL.

Fixes the NULL dereference a few lines lower (where it gets the object type).

cfoch/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,ylatuya/gstreamer,ensonic/gstreamer,mparis/gstreamer,lubosz/gstreamer,magcius/gstreamer,centricular/gstreamer,mrchapp/gstreamer,krichter722/gstreamer,magcius/gstreamer,drothlis/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lubosz/gstreamer,ylatuya/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,mrchapp/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,ahmedammar/platform_external_gst_gstreamer,Lachann/gstreamer,collects/gstreamer,krichter722/gstreamer,ylatuya/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,cablelabs/gstreamer,collects/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,magcius/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,drothlis/gstreamer,mparis/gstreamer,collects/gstreamer,cablelabs/gstreamer,Distrotech/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,mrchapp/gstreamer,lovebug356/gstreamer,collects/gstreamer,centricular/gstreamer,surround-io/gstreamer,surround-io/gstreamer,krichter722/gstreamer,ylatuya/gstreamer,mparis/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,ahmedammar/platform_external_gst_gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,magcius/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,drothlis/gstreamer,StreamUtils/gstreamer,ahmedammar/platform_external_gst_gstreamer,justinjoy/gstreamer,Distrotech/gstreamer,centricular/gstreamer,cfoch/gstreamer,ahmedammar/platform_external_gst_gstreamer,justinjoy/gstreamer,krieger-od/gstreamer,krichter722/gstreamer,ahmedammar/platform_external_gst_gstreamer,drothlis/gstreamer,cfoch/gstreamer,lovebug356/gstreamer,mrchapp/gstreamer,krieger-od/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,magcius/gstreamer,ensonic/gstreamer,ylatuya/gstreamer,centricular/gstreamer,drothlis/gstreamer,cablelabs/gstreamer,mparis/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,justinjoy/gstreamer,Distrotech/gstreamer,Lachann/gstreamer,lubosz/gstreamer,jpakkane/gstreamer,collects/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,Distrotech/gstreamer,centricular/gstreamer,mrchapp/gstreamer,ensonic/gstreamer,Lachann/gstreamer,lubosz/gstreamer,jpxiong/gstreamer