Low: tools/findif: sanitize SearchUsingRouteCmd a bit

Not knowing how the input SearchUsingRouteCmd generates and subsequently
processing [*], only the most conservative changes are contained in this
patch.

First, there was a futile assigment that would only be valid if
commented out strsep calls were in effect. Due to this, the assigment
was also only commented out.

Next, return value used to be taken from the first strtok in sequence,
only to be overwritten by another one.

Finally, if strtok unexpectedly returned NULL (due to input format other
than expected), it was not handled gracefully.

• "route -n get <addr>" is not supported in RHEL neither in Fedora

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>