gst/gstplugin.c: Print error debug message if plugin description fields that should...

gst/gstplugin.c: Print error debug message if plugin description fields that should be set are NULL.

Original commit message from CVS:
* gst/gstplugin.c: (CHECK_PLUGIN_DESC_FIELD), (gst_plugin_load_file):
Print error debug message if plugin description fields that should
be set are NULL.
* gst/gstregistrybinary.c: (gst_registry_binary_save_const_string):
Don't crash if the string to serialise is NULL (it really should
not be, but apparently this used to work with the xml registry ...).