imagefreeze: avoid assertion when using accept caps query
imagefreeze: avoid assertion when using accept caps query

This query must receive a fixed caps, so imagefreeze should
fixate its framerate before sending the query downstream.

https://bugzilla.gnome.org/show_bug.cgi?id=686837

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good