gst/wavparse/gstwavparse.c: Set variable to NULL after freeing it to prevent double...

gst/wavparse/gstwavparse.c: Set variable to NULL after freeing it to prevent double frees or make failures by another...

Original commit message from CVS:
Based on a patch by:
Victor STINNER <victor dot stinner at haypocalc dot com>
* gst/wavparse/gstwavparse.c: (gst_wavparse_stream_headers):
Set variable to NULL after freeing it to prevent double frees
or make failures by another use of it afterwards more obvious
and fix use of it after the freeing.