s4:torture: Initialize pointer with NULL

s4:torture: Initialize pointer with NULL

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/source4/torture/rpc/samsync.c:342: var_decl: Declaring variable ""dom_sid"" without initializer.
samba-4.20.0rc2/source4/torture/rpc/samsync.c:385: uninit_use_in_call: Using uninitialized value ""dom_sid"" when calling ""dom_sid_dup"".
  383|   	}
  384|   	if (samsync_state->domain_handle[database_id]) {
  385|-> 		samsync_state->sid[database_id] = dom_sid_dup(samsync_state, dom_sid);
  386|   	}
  387|"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>