Henry Wilkes authored Jan 27, 2020 and Jan Schmidt committed Feb 12, 2020

Keep the ANY caps empty internally when appending and merging
caps/structures. Previously, an ANY caps could end up containing
internal structures, which could be fetched by the user, and gave the
caps a non-zero length.

Also, made sure that `gst_caps_set_features_simple` frees the features
if caps is empty.