src/flac.c: Fix another memory leak
src/flac.c: Fix another memory leak

When the FLAC decoder was passed a malformed file, the associated
`FLAC__StreamDecoder` object was not getting released.

Closes: https://github.com/erikd/libsndfile/issues/233

libsndfile/libsndfile,libsndfile/libsndfile,erikd/libsndfile,libsndfile/libsndfile,evpobr/libsndfile,erikd/libsndfile,erikd/libsndfile,erikd/libsndfile,evpobr/libsndfile,evpobr/libsndfile,libsndfile/libsndfile,evpobr/libsndfile,evpobr/libsndfile,erikd/libsndfile,libsndfile/libsndfile