Fix: bt2: autodisc: remove thread error while inserting status in map

Fix: bt2: autodisc: remove thread error while inserting status in map

If something fails in `bt_bt2_auto_discover_source_components`, we
append an error cause and we go to the error label.  This function
returns a bt_value map containing the return status code and the auto-discovery results, if it was successful.
So we then run into:

	if (result) {
		insert_entry_status = bt_value_map_insert_signed_integer_entry(result, "status", status);
		if (insert_entry_status != BT_VALUE_MAP_INSERT_ENTRY_STATUS_OK) {
			BT_VALUE_PUT_REF_AND_RESET(result);
			PyErr_NoMemory();
		}
	}

However, since there is an error on the current thread, we fail this
precondition:

    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:64 Babeltrace 2 library precondition not satisfied.
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:66 ------------------------------------------------------------------------
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:67 Condition ID: `pre:value-map-insert-signed-integer-entry:no-error`.
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:69 Function: bt_value_map_insert_signed_integer_entry().
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:70 ------------------------------------------------------------------------
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:71 Error is:
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:73 API function called while current thread has an error: function=bt_value_map_insert_signed_integer_entry
    06-30 11:27:11.697 3948806 3948806 F LIB/ASSERT-COND bt_lib_assert_cond_failed@assert-cond.c:76 Aborting...

Change the function to temporarily remove the current thread error,
while inserting the status in the map.  In the unlikely event where the
insertion fail because of a memory error, then we just release the error
and it gets lost.

Change-Id: I1b54fb7c8cb0f719fee867e7385dd6a3949cbde4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8512
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>