gst/gstbin.c: Use an iterator to set the clock and the index so that we can release the object lock appropriately. Fi...

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_set_index_func), (gst_bin_set_clock_func),
(gst_bin_change_state_func):
Use an iterator to set the clock and the index so that we can release
the object lock appropriately. Fixes #566393.