isisd: fix crash when configuring the circuit type for the interface.

isisd: fix crash when configuring the circuit type for the interface.

1. When both Router A and Router B are configured with "is-type level-1," the area->is_type will be assigned the value IS_LEVEL_1, and circuit->is_type will also be assigned the value IS_LEVEL_1.
2. Configuring the circuit type "isis circuit-type level-1-2" for the interface of Router A will inadvertently call lib_interface_isis_circuit_type_modify to assign circuit->is_type the value IS_LEVEL_1_AND_2. This causes the hello packets reception and transmission, as well as the reception of LSP/SNP packets, to check circuit->is_type, allowing the level-2 hello packets to be sent and received normally, and level-2 LSP/SNP packets to be received normally.
3. When Router B modifies the configuration to "is-type level-2," and Router A and Router B establish a level-2 neighbor relationship, Router B sends level-2 LSP packets to Router A. Upon receiving these, Router A calls isis_spf_schedule to calculate the level-2 SPT, which results in accessing a null pointer.
When defining the behavior of the ISIS router, the call to isis_area_is_type_set will check that area->is_type is not IS_LEVEL_1_AND_2, and it disallows circuit->is_type_config from overriding circuit->is_type. Therefore, when configuring the circuit type for the interface of Router A, it should also check that area->is_type is not IS_LEVEL_1_AND_2 and disallow circuit->is_type_config from overriding circuit->is_type.

Signed-off-by: zhou-run <166502045+zhou-run@users.noreply.github.com>