gconv: Check reference count in __gconv_release_cache [BZ #24677]

This fixes a regression introduced in commit
7e740ab2 ("libio: Fix gconv-related
memory leak [BZ #24583]").

__gconv_release_cache is only ever called with heap-allocated
arrays which contain at least one member.  The statically allocated
ASCII steps are filtered out by __wcsmbs_close_conv.