ximagesrc: remove pointless g_return_val_if_fail()
ximagesrc: remove pointless g_return_val_if_fail()

ximage won't ever be NULL here because the dispose
function is called via ximage->dispose().

loshca/gst-plugins-good,loshca/gst-plugins-good,vatavuserban/gst-plugins-good,BigBrother-International/gst-plugins-good,stfl/gst-plugins-good,sh0/gst-plugins-good,shelsonjava/gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,BigBrother-International/gst-plugins-good,pexip/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,veo-labs/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,loshca/gst-plugins-good,chamois94/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,froggatt/gst-plugins-good-m,stfl/gst-plugins-good,sebras/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,BigBrother-International/gst-plugins-good,sebras/gst-plugins-good,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,sh0/gst-plugins-good,pexip/gst-plugins-good,veo-labs/gst-plugins-good,stfl/gst-plugins-good,sebras/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,GrokImageCompression/gst-plugins-good,vatavuserban/gst-plugins-good,chamois94/gst-plugins-good,hizukiayaka/gst-plugins-good,ikonst/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,rawoul/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,froggatt/gst-plugins-good-m,sh0/gst-plugins-good,Kurento/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,hizukiayaka/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,froggatt/gst-plugins-good-m,vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,kittee/gst-plugins-good,pexip/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,GStreamer/gst-plugins-good,loshca/gst-plugins-good,sh0/gst-plugins-good,sebras/gst-plugins-good,shelsonjava/gst-plugins-good,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,veo-labs/gst-plugins-good,BigBrother-International/gst-plugins-good,kittee/gst-plugins-good,chamois94/gst-plugins-good,veo-labs/gst-plugins-good,kittee/gst-plugins-good