Fix J in agenda when clock out hook clocks in again

Fix J in agenda when clock out hook clocks in again

* org-clock.el (org-clock-out): Do not delete the current clocking task
when org-clock-out-hook clocks in another task

My clock out hook keeps the clock running by automatically clocking in
a new task (the parent task, or the default task).  This sets a new
clocking task which was then clobbered at the end of org-clock-out
so that J in the agenda would return

    "No running clock, use `C-c C-x C-j' to jump to the most recent one"

We now detect that another task is clocked in and skip clearing the
org-clock-current-task variable used by the agenda to determine if
the clock is currently running.