mbimcli-intel-tools: parse trace command & value in a more straightforward way

Don't conditionalize setting trace_command and trace_value on split substrings being non-NULL. It makes a static analysis tool think they might be used uninitialized. That can-not happen, because the substrings can in fact never be NULL.

Let's keep the check in form of an assert (perhaps to guard against a possible glib bug, etc.).