lib: de-duplicate "no error" assertion in add port functions

Add BT_ASSERT_PRE_NO_ERROR_FROM_FUNC calls in
bt_component_add_output_port and bt_component_add_input_port, and remove
the equivalent calls in their callers.  This moves the assertion check
next to the other existing assertions.

Change-Id: Ia251510ce2b6ba369cfc1f4afae50e8e46060957
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10036
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>