input: es_out: fix non-matching main_clock bus

Fix the following assertion in some cases multiple programs are used by the es_out, leading to a mismatch between the main_clock bus being locked and the main_clock bus creating the input vlc_clock.

Assertion failed: (vlc_mutex_held(&main_clock->lock)), function vlc_clock_main_CreateInputMaster, file clock.c, line 998.
FAIL test_input_es_out (exit status: 134)

Co-authored-by: Sergio Ammirata

Reported that some US feeds with mpegts were triggering the assertion on vlc_clock_main_CreateInputMaster() call.

Superseeds !6884 (closed)