s3:registry: Avoid possible double frees

s3:registry: Avoid possible double frees

Found by Covscan.

"Error: USE_AFTER_FREE (CWE-416):
samba-4.20.0rc2/source3/registry/reg_perfcount.c:309: freed_arg: ""_reg_perfcount_multi_sz_from_tdb"" frees ""*retbuf"".
samba-4.20.0rc2/source3/registry/reg_perfcount.c:313: double_free: Calling ""_reg_perfcount_multi_sz_from_tdb"" frees pointer ""*retbuf"" which has already been freed.
  311|   	for(i = 1; i <= base_index; i++)
  312|   	{
  313|-> 		buffer_size = _reg_perfcount_multi_sz_from_tdb(names, i*2, retbuf, buffer_size);
  314|   	}
  315|   	tdb_close(names);"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Signed-off-by: Martin Schwenke <mschwenke@ddn.com>