s4:torture: Initialize pointer with NULL

s4:torture: Initialize pointer with NULL

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/source4/torture/smb2/durable_open.c:1447: var_decl: Declaring variable ""tree2"" without initializer.
samba-4.20.0rc2/source4/torture/smb2/durable_open.c:1492: uninit_use_in_call: Using uninitialized value ""tree2"" when calling ""smb2_util_unlink"".
 1490|   		}
 1491|
 1492|-> 		smb2_util_unlink(tree2, fname);
 1493|
 1494|   		talloc_free(tree);"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>