decodebin3: Add missing INPUT_LOCK() before ensure_input_parsebin()

Alicia Boya García requested to merge ntrrgc/gstreamer:20230905-input-lock into main

ensure_input_parsebin() has a top comment saying it must be called with INPUT_LOCK taken, but 2 out of 3 usages of the function call it without taking that mutex.

This patch adds locking in these two remaining usages.

This is a clean-up patch -- as of writing I haven't identified any user-facing issue from this missing locking, and I want to see if the CI catches any regressions.

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