coresight: checking for NULL string in coresight_name_match()

coresight: checking for NULL string in coresight_name_match()

Connection child names associated to ports can sometimes be NULL,
which is the case when booting a system on QEMU or when the Coresight
power domain isn't switched on.

This patch is adding a check to make sure a NULL string isn't fed
to strcmp(), something that avoid crashing the system.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>