matroskamux: Fix segmentation fault
matroskamux: Fix segmentation fault

Use g_strcmp0 instead of strcmp to avoid null pointer exception.

Change-Id: I6f15b3be507a9dd992ed2c4a96e623161bf741f0

Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good