ext/flac/gstflacenc.c: If seeking failed return the appropiate return value to FLAC.

ext/flac/gstflacenc.c: If seeking failed return the appropiate return value to FLAC.

Original commit message from CVS:
* ext/flac/gstflacenc.c: (gst_flac_enc_seek_callback):
If seeking failed return the appropiate return value to FLAC.
Otherwise it thinks seeking was successfull and tries to rewrite
parts of the headers which then get appended to the output.