In some cases, decodebin3 will send us incomplete caps (not containing
codec_data), and then a GAP event, which will force a negotiation.
This segfaults due to a null pointer deref because self->input_state
is NULL.
The only possible fix is to avoid negotiating when we get incomplete
caps (to avoid re-negotiationg immediately afterwards, which isn't
supported by some muxers), but also set as much input state as
possible so that a renegotiation triggered by a GAP event can complete
successfully.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7634>
80bb69ba66