Fix the bound size and the variable name

It will not end when other words appeared because of the wrong bound and variable,
rule_map->length will exceed the actual length in the rule_map_new function,
it will lead to crash in the rule_map_validate function because of strcmp(NULL, str).

Test: 1.add "user=shell doman=system_app" to private/seapp_contexts
      2.exec "checkseapp private/seapp_contexts" and it will not be crashed

Change-Id: I600206448b38cf2c9b61f9141b40f920b05696c8
Signed-off-by: liwugang <liwugang@xiaomi.com>