gst/gststructure.c: Always check UTF-8 conformance of structure strings and not only...

gst/gststructure.c: Always check UTF-8 conformance of structure strings and not only if the debugging system is enabl...

Original commit message from CVS:
* gst/gststructure.c: (gst_structure_id_set_value):
Always check UTF-8 conformance of structure strings and not only
if the debugging system is enabled; reasoning: the behaviour of
the actual code shouldn't really change depending on whether the
debugging system is enabled or not (#508291).