Fix: src/lib/graph/graph.h: do not override `comp_status` value

This made, for example, bt_graph_run() return
`BT_GRAPH_RUN_STATUS_ERROR` when the sink component's "graph is
configured" method returned
`BT_COMPONENT_CLASS_SINK_GRAPH_IS_CONFIGURED_METHOD_STATUS_MEMORY_ERROR`,
therefore not percolating the correct status.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic89d9315f8bc1c3226a14955c9e6419462d5e588
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1829
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>