decodebin3: Fix slot input linking when the associated stream has changed

Setting the input field on the empty slot prevents future linking of it and will result in flow errors later on.

This was observed in WebKit's MediaStream source element, when it changes the caps on one of its associated streams, from an encoded format to a raw video format. The associated stream-id on the sticky stream-start event doesn´t change, but the element creates a new GstStream with a different ID and sets it on the stream-start event. Stream parsing is disabled in urisourcebin, so decodebin3 handles the parsing. Without this patch we would end-up with unlinked pads in decodebin3 after switching to the raw video format.