lib:util: Move open() of /dev/null into the if-clause

lib:util: Move open() of /dev/null into the if-clause

"Error: RESOURCE_LEAK (CWE-772):
samba-4.20.0rc2/lib/util/close_low_fd.c:40: open_fn: Returning handle opened by ""open"". [Note: The source code implementation of the function has been overridden by a user model.]
samba-4.20.0rc2/lib/util/close_low_fd.c:40: var_assign: Assigning: ""dev_null"" = handle returned from ""open(""/dev/null"", 2, 0)"".
samba-4.20.0rc2/lib/util/close_low_fd.c:52: overwrite_var: Overwriting handle ""dev_null"" in ""dev_null = open(""/dev/null"", 2, 0)"" leaks the handle.
   50|   	}
   51|
   52|-> 	dev_null = open(""/dev/null"", O_RDWR, 0);
   53|   	if (dev_null == -1) {
   54|   		dev_null = open(""/dev/null"", O_WRONLY, 0);"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>