gst/base/gstcollectpads.c: Guard public API with assertions.

gst/base/gstcollectpads.c: Guard public API with assertions.

Original commit message from CVS:
* gst/base/gstcollectpads.c: (gst_collectpads_set_function),
(gst_collectpads_add_pad), (gst_collectpads_remove_pad),
(gst_collectpads_is_active), (gst_collectpads_collect),
(gst_collectpads_collect_range), (gst_collectpads_start),
(gst_collectpads_stop), (gst_collectpads_peek),
(gst_collectpads_pop), (gst_collectpads_available),
(gst_collectpads_read), (gst_collectpads_flush):
Guard public API with assertions.
* gst/gstpad.c:
Fix docs for gst_pad_set_link_function().