Fix -1 issue when fan collection size is 0.

When fan collection size is 0, the MinNumNeeded shown -1
(4294967295/0xffffffff) but not 0.

    "Redundancy": [
        {
            "@odata.id": "/redfish/v1/Chassis/WC_Baseboard/Thermal#/Redundancy/0",
            "@odata.type": "#Redundancy.v1_3_2.Redundancy",
            "MemberId": "Tach",
            "MinNumNeeded": 4294967295,
            "Mode": "N+m",
            "Name": "Tach",
            "RedundancySet": [],
            "Status": {
                "Health": "OK",
                "State": "Enabled"
            }
        }

Tested:
"MinNumNeeded" is 0 correctly.
https://bmc_ip/redfish/v1/Chassis/WC_Baseboard/Thermal
    "Redundancy": [
        {
            "@odata.id": "/redfish/v1/Chassis/WC_Baseboard/Thermal#/Redundancy/0",
            "@odata.type": "#Redundancy.v1_3_2.Redundancy",
            "MemberId": "Tach",
            "MinNumNeeded": 0,
            "Mode": "N+m",
            "Name": "Tach",
            "RedundancySet": [],
            "Status": {
                "Health": "OK",
                "State": "Enabled"
            }
        }

Change-Id: I8bf01dacd705e5309c161c5f04289d2df45ca583
Signed-off-by: Kuiying Wang <kuiying.wang@intel.com>
diff --git a/redfish-core/lib/sensors.hpp b/redfish-core/lib/sensors.hpp
index 212a4a4..f12bbe0 100644
--- a/redfish-core/lib/sensors.hpp
+++ b/redfish-core/lib/sensors.hpp
@@ -1208,6 +1208,10 @@
                                     }
                                 }
 
+                                size_t minNumNeeded =
+                                    collection->size() > 0
+                                        ? collection->size() - *allowedFailures
+                                        : 0;
                                 nlohmann::json& jResp =
                                     sensorsAsyncResp->res
                                         .jsonValue["Redundancy"];
@@ -1220,8 +1224,7 @@
                                           std::to_string(jResp.size())},
                                      {"@odata.type",
                                       "#Redundancy.v1_3_2.Redundancy"},
-                                     {"MinNumNeeded",
-                                      collection->size() - *allowedFailures},
+                                     {"MinNumNeeded", minNumNeeded},
                                      {"MemberId", name},
                                      {"Mode", "N+m"},
                                      {"Name", name},