decodebin3: Add missing INPUT_LOCK() before ensure_input_parsebin()
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.