gst: check non-null before dereference
gst: check non-null before dereference

It is possible to use gst_deinit() without registering the base
classes. For example, when using gst_init_get_option_group() and
call the program with an invalid parameter. In that case,
gst_deinit() will lead to a segmentation fault, since there is a
dereference to a pointer that is null.

This patch validates if the type is non-null before dereferencing
it.

https://bugzilla.gnome.org/show_bug.cgi?id=781914

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer