Fix resource leakage detected by Coverity.

lib/zip_file_set_comment.c

diff --git a/lib/zip_file_set_comment.c b/lib/zip_file_set_comment.c
index 3df697a..e7f9590 100644
--- a/lib/zip_file_set_comment.c
+++ b/lib/zip_file_set_comment.c
@@ -87,6 +87,7 @@
         if (e->changes == NULL) {
             if ((e->changes=_zip_dirent_clone(e->orig)) == NULL) {
                 _zip_error_set(&za->error, ZIP_ER_MEMORY, 0);
+		_zip_string_free(cstr);
                 return -1;
             }
         }