Update g72x.c
Update g72x.c

Fixing signed integer overflow as reported in https://github.com/libsndfile/libsndfile/issues/757 . A similar fix was applied before https://github.com/libsndfile/libsndfile/pull/818
libsndfile/libsndfile,libsndfile/libsndfile,libsndfile/libsndfile,libsndfile/libsndfile,libsndfile/libsndfile